Disclaimer: This is in no way meant to sounds 'dick-ish' or judgmental or anything like that, I just was in a similar place a few years ago when I was in school, and found myself in a similar situation at the start of this year for some time. Just a harsh truth that I need a reminder of now and then...
So I've suffered from anxiety and depression since about the 3rd grade. It can be debilitating at times, mix in a natural laziness as well and I tend to let it hold me back quite a bit.
Anyways, I noticed something in this last comment of yours that I have a recurring problem with - I allow myself to perform below my potential, and in all aspects of my life. In relationships, in jobs, in how I even treat myself. I know what I'm capable of accomplishing, but I will give up at the first sign of adversity or difficulty, or really, anything that I need to actually work for. I've abandoned a lot of things I've set out to accomplish just barely after starting, and then later feel depressed or angry or whatever when I'm not where I want to be.
The problem I struggle with is accountability. I'll chock it up to the anxiety or depression, or that I wasn't given what I needed to accomplish my goals, etc etc etc. I was okay with that for a long time, and constantly copped out of accepting responsibility and blaming other people or circumstances on the outcome of where I was at that time.
This is where you come in - 'I just need some tools to do better.' I understand what you mean. What you want to accomplish with your life to be better goes against everything your natural instincts have always told you to do or is acceptable, and you feel like you need tools or help in order to do that.
'...I eat crappy food and feel addicted.' again, another cop out. You don't want to eat like crap anymore and you want to lose weight or whatever, but this addiction is holding you back.
Before you can move forward and start on the road to being the person you want to be, you need to take a harsh look at yourself and take accountability for where you are right now. This can be pretty tough and I don't mean to make it sounds like I'm saying 'just do it', and it certainly doesn't happen overnight (it's also something you need to continually work on or else you'll tend to 'relapse' and start blaming other things for your situation again).
Thinking you need those tools to be a better is really whats holding you back - saying that you just need some tools in order to be better better relieves you of the burden, and places the responsibility elsewhere. Same with the idea of addiction. It's okay to admit that you just really like eating crappy food. I love crappy food. Haha. One of my favorite past times is finding a good movie and sitting down with a ton of crappy food and just binge eating. But if it's not inline with your overall goal of weight loss or being healthier, then you need to own it.
It's really not all bad though - where this harsh realization originates is also where you can find the best motivation to keep going, and stick on that road of becoming a better you. When you own the negative outcomes of your actions, you also see that you own the positive outcomes. You can always fall back on knowing that with a little perspective, or a kick in the ass, your goals really aren't out of reach. The way I look at it, is not to make it into this mountain of a task that you need to overcome to get to your goal.
One kind of stupid idea that makes it easy for me is this - think about life like the old school super mario games, regardless of what you do, life will always be moving in the same direction. It doesn't matter how much you kick or scream, it's just the inevitability of our reality. You can't go back and choose to go a different route, or start over from the beginning, and you need to accept that. But that's okay. Its behind you now. What you can do is choose to take a step in the direction you want to go, and let the momentum of good decisions help take it from there.
If you find yourself driving in the opposite direction of where you want to go, do you keep going in that direction hoping you'll just end up where you wanted to be? Of course not. Well it's similar to your life... turn that bitch around and start heading in the direction you WANT to be going, but enjoy the ride. You will need to work for it, and you will face some difficulty, and it won't be easy.... but it isn't impossible either. But setting yourself up for success and just enjoying the ride is how you not only get to your goal, but it's how you find what you really love in life.
I apologize for the ridiculously long rant, but I'm also kind of reminding myself about all of this. I also didn't mean to sound preachy or like I've got it all figured out. I don't. I just know that putting things into perspective and getting out of my own way is always the first step that I need to take, or else I just end up in the wrong direction again.
Regardless - good luck, and remember that you only just turned 21. A year from now you're going to be a completely different person, and a year after that, you'll be someone else. Enjoy this time while you can, and just keep working on taking small steps in the right direction!