tl;dr: below contains stupid, unedited, theories. Do not read if you like quality content.

All content blacked out for spoilers.

As a young kid in a strange situation, with odd beings attacking me?

I'd end up growing up in the RUINS, and who knows what ideals someone in that situation would end up as after being raised by a pacifist goat-person. Early me was very interested in science-type stuff, so I'd likely end up doing a lot of bug catching, perhaps end up as a "scientist of the creatures of the ruins". Perhaps I would seek out, or find enough knowledge of magic, soul, or so on, to do experiments regarding those things, but I am unsure as those topics aren't covered in the game.

I absolutely wouldn't ever risk leaving security, walking deep into some woods, especially at a young age. However, there would be a lot of crying, homesickness, and other issues along the way.

In my current state of mind?

I'd likely take some time directly speaking to Toriel, asking a whole lot of questions. I'm not too sure if I'd attempt to leave the ruins knowing that the king of the area I'd be leaving into seeks to kill me. I'd likely end up trying to build some sort of ladder to climb back out of the hole I fell into.

Assuming I failed at that, I would absolutely get bored and restless with no internet, one person to talk to, no electricity, and so on. I'd look into the whole "magic" thing, likely read a whole lot of books, and eventually try to leave the area.

If I were put into the situation where the door was about to be closed on me, as in what happened in the game, I'm not sure how I'd react. Likely I'd try and reason, saying that I need time to figure things out, before I would set off, and things like that, trying to "put off" the event of leaving long enough so that Toriel is able to handle it more, and so I could learn more about the world of Undertale.

If I learned that it takes both a monster, and a human soul to exit the barrier, I'd probably be put off quite a bit, and if I learned you have to have a boss monster die to get out, I would be very conflicted over if to seek and kill Asgore or just stay hidden away, especially considering that the boss monsters do not age, and I couldn't just wait for one to die of natural causes. Likely I would tell myself the right course of action is murder, but would also reason that I am not powerful, dedicated, or of the correct mindset to accomplish such a goal. I'd also likely consider the effects on murdering the king on the state of the underground.

I would view Asgore as a person who had killed six children, and would assume there is no talking down a monster like that

I figure not much would be different in the whole meeting Sans/Papyrus. The fact sans would assure you that there was no real harm, and the derpyness of Papyrus would have me playing along, and I'd likely end up avoiding capture in much the same way which happened in the game, minus any form of flirting.

Say I learned of the lab, royal scientist, and all that, and I assumed it was a good idea to seek them out to get more information about monsters, souls, and so on. This would lead me to leaving snowden/the ruins, and seeking out the hotlands, putting me into the waterfall area, where I would meet Undyne.

I'd die to laser spears here, almost certainly. Perhaps I'd be prepared by bringing along some friends, if I had made any, and if they were around, I would be able to survive.

Assuming I survive

I'd continue past this point, likely not actually befriending Undyne, unless the "friends" I brought along with me directly caused that to occur. However, I would continue into the hotlands, where I would meet Alphys. As I did not seek to get to the core, and instead only was looking to just speak to Alphys directly, to learn more about the world, the metton fights wouldn't be significant. I would be significantly creeped out by the fact I was being watched constantly since leaving the ruins, and would likely wonder why I hadn't had my position been reported up to that point, assuming it hadn't and Undyne hadn't come back and kill/captured me. (Should that have happened any time before that, skip to the "Assuming I die" section.) I would likely ask a whole lot of questions, and get kicked out of the lab for making Alphys question herself due to the determination experiments, and my own questioning being similar.

The most convenient reaction would be Alphys breaking down and assuming that I am asking the questions I have thanks to me knowing the determination experiments existed, and I was being passive aggressive. I would be pointed down to the true lab, and would likely not go down there unless with Alphys to guide, reducing the need to fight ghosts/aglomerates.

After being thoroughly freaked out by the experiments which were done, and would likely be incredibly put off the idea of doing experiments with souls, I'd go one of two ways.

Either learn some stunning new fact, and eventually try to research how to "absorb" a monsters soul without it being a boss monster. If I did this, I would find a monster about to pass away, perform the ritual, and having that extra soul, I'd find a way to climb back up the hole in the ruins (possibly breaking into the ruins somehow), and leave that way.

If I did not, I'd likely end up hiding away, with friends assisting me not being found by Asgore, until I was inevitably found out. At this point, Asgore would likely find and kill me, at which I would go to the "Assuming I die" section, or I would possibly talk Asgore out of his actions once learning that he really isn't into the concept of killing children, at least assuming that I have friends around to help with that as well.

Assuming I die

However, if I was in the situation of being able to reset, I would wake up, incredibly confused, at either the beginning of the game, or the latest save-point I stumbled into by accident.

I'm going to assume it was the first point in the game, as I wouldn't pay any attention to savepoints if I didn't know they existed, and it makes it convenient to have this section tie up all the points at where I would actually die.

If a reset actually would occur, I would be confused in an incredibly fundamental way. I'd likely not leave that bed of flowers for quite some time, doing nothing but being amazed with what, I assumed, was a teleportation, and scared out of my mind having just been burned/lacerated/killed. Having "felt" this ability to reset to a previous save, the first thing I would do is toy with the ability. Move a plant to somewhere, reset, see if it moved as well. Once I learned that I was literally turning back time, reseting everything I had done, I would be somewhat relieved, but somewhat angry that everything I had done was meaningless.

I would also be concerned with being trapped in limbo for my entire life, or relieved by the concept of never dying.

This is something I'm not too sure how I'd react to. I'd absolutely get philosophical/scientific. What am I doing? Is there a new timeline made? What is a reset? Likely, I'd be very obvious about what just happened, not really bothering to hide what I had been talking about, and would likely attempt to prove my new abilities by repeating my actions once through, and telling someone what they are about to say, or writing it on a card. I'd be hesitant to do it at first, cautious of the consequences, but if it's what it took for someone to believe me, I'd likely try it, especially having already done a reset.

This would likely end up with me directly mentioning what is going on to Sans.

Honestly, how it would go from there, I have no idea, either I spend ages experimenting with the ability in tandem with Sans, who seems to have already done research on the subject to have so much knowledge of it, or Sans doesn't help at all, and I end up using the reset as a way to just get back to where I was before, where I figure out a way to directly accomplish my goals, as happens in the game, returning me right back to the point where I die, but with the keen ability to always do the right thing to progress forward. Likely, I'd take actions which end up killing people, but always by accident, putting something somewhere that makes someone trip rather than stabbing them.

Through all this, flowey would likely be quite the enemy. As I grow to understand resets, I would likely start getting enough knowledge of who/what this creature is to start to directly fight it/him. I am unaware what he would try to do to cause things to go off, so I largely ignore this whole thing in this post, because it's too long already.

Either way, with this reset ability, I continue to do resets until I either stop dying. If I learn that you can't reset outside of the underground, I likely wouldn't ever leave, opting to do as flowey did, living long life after long life until I go insane and get unpredictable, or get usurped by flowey unless I find a way to quickly and easily kill him shortly after any reset (the first year or so).

Things I forgot

Flowey would play a much larger factor. I'd likely be incredibly creeped out and scared by him at first, and if he doesn't appear, I'd forget about him in time. Likely, he would seek to end my ability to create resets somehow, but clearly not in the same way he tried to do this for frisk, as I wouldn't be stupidly seeking out the person who wants to kill me.