This hits really close to home, so I'll tell you what I think, but keep in mind I'm only relating to my own experiences. I don't know that much about your situation, so this might be not helpful at all.

I have done some things where afterwards I couldn't believe my naivety or lack of good judgement - years after I can still feel uncomfortable thinking about. Sometimes I'd get lucky and nothing would happen, as a result of my ill judgement, but I'd know it was close. It would always make me feel like shit, and I don't think it's unreasonable - I almost got raped once, and while I know there would be no excuse for what the guy might have done and he would be to be blamed for it, I also could have easily avoided the situation. But there were a few things that helped in the short and in the long run (and I haven't done anything particularly stupid in a long while). Firstly, it's in the past, so you can't do anything to take it back. Accept that it happened, that it made you feel bad, that the guy is a piece of shit that took advantage of you, and also that yes, you could have protected yourself better from it. However, even though you can't change what happened, you can decide how to act responsibly right now. Apart from the day after pill, which you took, you should consider getting PEP to prevent HIV infection (if it's still under 72h after the intercourse) and you should also get other STD tests done.
What helps me every time I fuck something up (not just talking about bad judgement, but anything, including work stuff of anything else) is also to just spend time doing something productive and somehow valuable (for myself, or others, doesn't matter). This way I'm not too focused about the shitty thing, because I'm also very happy with myself after doing whatever it is that I did to make myself feel better. And lastly, I am trying very hard to filter through my excitement. If something seems amazing (and I notice that my reaction is more extreme than maybe it should be) I'm just trying to focus on something else and think what it is that I'd advise a friend to do and try to follow my own advice. It tends to work, because it feels good to me to know I'm not a complete slave to my emotions. I can't control them, but I'm doing my best to not have them rule me.

And so, if we're talking about emotions, here is the other part - the not being able to get over him thingy. I'd be inclined to say you're likely not into this actual person (especially considering you recognise that he's a dick), but what you can't get over might be the fantasy, the possibility of what it might have been (even if just for the night!). Or maybe you don't want to accept that this amazing beginning was an illusion and a lie - because how will you know in the future if you can trust someone? Or because it makes you feel vulnerable if you think you're easy to manipulate. Or maybe it made you realise how much you crave closeness, and then you were given it and it was taken away from you in a cruel manner.
Last year I spend a few months talking to one guy online and after we finally went on a date he flaked on me - despite the date being long, him being able to easily use an excuse to finish it and him later following up with an amazing message. I really liked that guy, although I knew from the beginning he wasn't as interested as I was. He wasn't a player, but it wasn't balanced in any way. I nevertheless got attached way too easily and it took me months to completely move on. I would regularly check his blog for like 2 months after it all ended, and I couldn't understand why I'm doing it. I knew I didn't know him at all really, I had many reasons to think it wouldn't work out anyway, and I didn't even feel all that infatuated - but I somehow couldn't let go. And then I understood - I was not thinking about this guy, I liked the fantasy, the possibility of finding someone I really liked and could be close with. I liked how, I don't know, supportive he seemed. And I think I just took it hard to be able to see what it is that my life is missing, and then have to get back to the sad reality. I guess in the end it just passed, soom after realising it the man in my bedtime PMS fantasy became faceless again. ;)