I don't even know why she's with me, I mean lets face it I'm a screwed up depressed teenager unemployed and Happy 1 moment then very very depressed the next.
She's with you because she wants to be, because she cares about you, otherwise she wouldn't be there regardless.
I was with someone who was an alcoholic who suffered from depression, he could be pretty awful to me sometimes, life was hectic and stressful with him, but i was with him because i loved him. No matter what he put me through, i still loved him, because i could see the person struggling behind all the other stuff going on.
We're no longer together, but i ask myself the same question of my current partner, 'why is he with me?' Again he'd answer the same thing, he's with me because he wants to be and because he loves me, just as i am, warts and all so to speak (horribe expression i know!, but you get the gist!).
I think sometimes we have to find the one positive in our lives, and sometimes that is the love of someone close to us. I'm not saying you need someone with you to have something worthwhile, because i was on my own for about 4-5 years bringing my son up and i was still self assured, but when you do have someone there for you it is special.
Try to relax and go with the flow; as you said you think things will go wrong so they do, it's a kind of self fulfilling prophecy! Why not try a more 'if things don't work out it'll be ok, and if they do even better' attitude? It takes a lot of effort to mentally think force a relationship down the toilet!, it takes a lot less to just let go and relax and see where things go, and it's a more enjoyable journey along the way 
It's tempting to place yourself at the bottom of the poo pile, that way if you're already down there no one can push you down there, there's no surprises that way, no unexpected hurt, if you're already at the bottom you can control the hurt, know when it's coming, control it. It's also not a great way to live, the view is much nicer at the top, yes there's risks, as there is everyday in life, but sometimes the risks pay off and you meet some truly wonderful people. At the bottom you just stay still, you cant move up or down. It took me a long time to 'climb up' and take some risks, and i'm so glad i did because i've met some brilliant people, and also my soul mate along the way. I'm not saying it's plain sailing, you always get people who are going to try and hurt you, as you do in any situation, but the good guys outweigh the bad ones a million times.
I'm not going to tell you drink isn't the answer because i'm guessing you've heard that a million times, and you alreasy know that. You're old enough to make your own decisions where that's concerned, just remember you've always got the choice xxxx
Thinking of you,
Sarah 