Please consider posting in our OCD community, this topic is discussed there all the time, the folks there are very used to the cubject and can give you advice. Even if you read through the threads, I think that will help.
http://www.medhelp.org/forums/Obsessive-Compulsive-Disorder-OCD-/show/231
You could be bisexual but prefer women. There's nothing wrong with that. There's nothing BAD about having these thoughts. If you've always liked women and are attracted to them, that's not going to change IF you find you COULD be attracted to men as well. Bisexuality is a real thing. You might find one day that you can be aroused by men but that doesn't mean you will suddenly wake up one day and like ONLY men. If you decide not to act on your bisexual desires--if you're perfectly happy with being attracted to women, then that's fine. If you end up experimenting one day with a man, that's also fine. You need to worry less about what these thoughts might "mean." You're young. Your naturally going to sexually explore ideas.
You need to stop worrying about labels and as if its gay vs straight. You also aren't going to learn anything by sitting in your house hiding from the world. I suggest seeing a therapist about the issue and talking through your fears and depression, especially if you're suicidal at times. This is a VERY VERY stupid thing to think about committing suicide over.
My advice (after speaking with a therapist, which I strongly urge) is to go out on a date with a girl. Kiss a girl. See how you feel. This very well may put your mind at ease. If you're attracted to her, then cool. If you occasionally have thoughts about guys, then that's cool too. Human sexuality is very fluid. It's not straight vs gay. Its not so rigid and so easily broken.