I've been dating a girl for 3 months now. We are both under 18 and she has recently told me about her past with other guys. She has slept with at least 5 other guys and this fact makes me very uncomfortable. I really like her and have very strong feelings for her, but I can't get past the number of guys she's been with - one of whom was a close friend of mine until recently when I started dating this girl. I know she suffered from depression and this could have affected her decisions. I'm really trying to get over this because I do love her, but I can't stop thinking about all the different people she was with before. I don't know if I should stay with her or leave. I keep telling myself it was her decision, and I shouldn't let it bother me, but it doesn't help. I've tried talking to her about it, but it's a touchy subject. She says she doesn't regret it, but admits it was stupid and our relationship will be different. I can't help but think that these are things she probably said to every other partner. I don't know what to do.
I think you do know the answer, but you don't want to face it. Please realize that I only have what you've written to go on, but I think the key point is that she says she has no regrets about her fornication. That tells you that she sees nothing wrong with having sex outside of marriage. The number of guys she has been in bed with without regret also tells you that she is not choosey about who she has sex with and that she isn't stable in her relationships. Likely she pushes things too far too fast and then changes her mind later.
There are many people who suffer from depression but who don't jump from bed to bed, so her depression isn't the reason for her promiscuity. If anything, her immorality is causing problems in her life and that is leading to her depression. Keep in mind that some woman use sympathy as a way to get guys to like them. Guys have a natural desire to rescue people and some women know how to use that to manipulate a man.
People don't change without good reasons, but her reasoning is that you are a different guy. The problem is that she is the same girl. That hints that in all these broken relationships, she blames the guy and doesn't see herself as contributing to the problem.
You are correct, you ought to be concerned. My guess is that if you continue with her, you will be guy number six. At five guys, she hasn't stayed with anyone for very long.