I have a girlfriend, but I'm gay, how can I resolve it?

First of all: I'm not a native speaker, I've been studying english just for 5 months and I'm writing it by myself without using any translator, so I'm sorry for my mistakes. I'm 17 years old and I have a girlfriend, but I'm gay. In the beginning I didn't know what I was doing, I think I was just testing myself, I thought I could date girls and change myself, you know what I mean? But now, I'm totally fucked up, because now I'm pretty sure about what I'm and I tried everything to break up with her without hurting her, but I can't. Firstly because I can't come out, I don't pay my bills yet and my parents won't accept me, secondly because I don't know how to do it,and thirdly because she loves me and I feel like a crap because I did it, I'm guilty, I'm selfish and weak, I did this and I can't turn back in time and to rub salt into the wound our relationship is getting serious, I met her parents and she met mine, we're getting close of sex and consequently I'm getting freak(not because I think I can't do it, I really can, but it'll become the things worst). I don't know what will be my next step, I'm afraid, I'm confused. Don't get me wrong, I've been through the phase of getting lamenting me because I'm gay and saying " Why did God choose me?", but I put myself in a bad situation and I don't know how to get me off, do you guys have any tip?Btw: I know I'm a monster, I know that I used her, but what could I do? I'm just a guy who the life selected to play unpleasant tricks on.“The pure and simple truth. Is rarely pure and never simple. What's a boy to do. When lies and truth are both sinful?” - John Green (I know it's cliche, but it defines everything).