Hello, and welcome to TeenHelp! =)
I'm sorry to hear about what's happening with your current crush. Women are so complicated, aren't they? =P Sometimes, they won't say what's really on their mind, and just beat around the bush or avoid the question altogether.
There could be several reasons why she's behaving this way. One, she may not like you in the romantic sense anymore. This does NOT mean that you did something wrong. Have you ever obsessed over something for a few weeks, only to wake up one day and realize that you don't care anymore? Some crushes are like that. People experience intense feelings of love for someone, but those feelings can't be maintained for long, and they "burn out" after a few weeks. Perhaps she feels that way, but doesn't want to hurt your feelings by telling you that she just wants to be friends.
Two, she may just be an extremely insecure person, and SHE doesn't think she's good enough for YOU. Unfortunately, no matter how much you convince her, she's probably going to keep feeling insecure and unworthy of being in a relationship with you. This is something she needs to deal with... perhaps by speaking to a psychological professional, because chances are these insecurities are affecting other areas of her life as well. Frankly, you need to find someone who is emotionally and mentally healthy... because if you date someone who isn't, then the relationship will also be unhealthy.
Three, she may have had bad experiences with boyfriends in the past, and she doesn't want to ruin your current friendship by starting a relationship that (in her mind) will end badly. Other than "being there" for her, in case she ever wants to talk about her old relationships and what went wrong, there isn't much you can do. Like her insecurities, this is something that she needs to deal with on her own. She needs to accept that not all relationships are going to end badly, and that it's worth giving you (or other men she may meet in the future) a chance.
Finally... I don't want to sound harsh, but have you ever heard the phrase "You must learn to love yourself before you can love another"? I can understand why you are depressed... I think just about anyone in your position would be... but what you need to realize is that your depression is both CAUSED by and CAUSING this problem. It's a perpetual cycle of self-loathing that is driving potential girlfriends away... so until you can address this depression, and learn to overcome it, you may continue to suffer from this problem. What I would like to suggest is that you talk to a school counselor, or even get a referral to see a therapist from your family doctor. Thinking about suicide is never a good thing... even if you don't think you'd ever act on it, it's something that is negatively affecting your life, and a counselor/therapist may be able to help you cope with the pain.
Remember, you are still young! You have your entire life ahead of you. =) Maybe people don't experience their first kiss, first date, first relationship, etc. until they are well into their 20's, 30's, even 40's! I'm not saying that you'll have to wait that long, of course. =P But I am saying that you WILL meet the right girl one of these days... and when you do, it'll be PERFECT. You'll just KNOW that she's the one, and you'll be so glad that you didn't rush into a relationship with someone who wasn't right for you, or during a time in your life when you weren't ready for one.
Good luck, and please feel free to keep in touch. We're all rooting for you!