What to do in my current situation? (again)

So 2 weeks ago this is what was going on:
loveforum.net/love-advice-forum/43104-what-do-my-current-situation.html
Our full story is there.

So, since then we'd been okay. We'd been talking online, talking on the phone, being optimistic etc.

But disaster has struck:
July 8: She's hanging out with her guy friend (the one who started our whole disaster, see link). & they kiss, passionately. She didn't feel guilty about doing it either, she said they didn't stop & described it as being "[like] sex with clothes on".
July 9: She goes on a date with this guy, under the pretense that they're just friends hanging out together. But as it happens they were kissing & being intimate on this [not so] friendly date.This night she's being kinda different, she seems guilty of something, but won't tell me what when I ask. So one thing leads to another & ask her if she kissed him (I was almost joking because I never thought she would) & she admitted she did. As expected I wasn't happy, at all. I asked her why, called her a few things, got really pissed off. She kept apologising but I didn't accept. Eventually I calm down a bit & tell her I want to forgive her, so long as she promises it won't happen again. She apologises & says she can't because she's certain it will happen again.
July 10: We talk all day (I'm a bit more relaxed, but still mad), she tells me she's sorry but admits she sees this guy & me in the same light, i.e we're of equal attractiveness etc. I tell her she has to choose between me or him & she says that she can't. Anyway one thing leads to another & she dumped me. But we're still friendly & she claimed she would change her mind on Monday (it's Sunday night now).
July 11: I spend all day crying & feeling sorry for myself but acting tough & friendly on Facebook.
Right now (July 11 night) she is with him, doing what I don't want to imagine.

So that's the story up 'til right now. I'm really nervous about tomorrow. I love her so much I think I will die if she breaks my heart in a face-to-face conversation.

What should I do? Should I forget about her? Is she more trouble than she's worth? Do I only feel like this because she is my first love? I've never felt so terrible before in my life.

Thank you so much for any help, it's much appreciated.

Heartbreak sucks but if you do get back with her, she will hurt you even more and will continue to do so. Don't take her back and yes deal with the pain which all us humans have to go through! she is not the one for you and obviously she doesn't respect you enough to not cheat. I find it astounding she still has the control in the relationship when shes the one who should be begging you to take her back!

Is she more trouble than she's worth? ABSOLUTELY.

I get that you have no real experience here & you've pointed out you have low self esteem & what not.. but.. this isn't the type of experience you need to be getting, & ultimately, she'll do more damage to your self esteem than good. (Especially if she's hooking up with some other dude & not even feeling guilty about it.)

Advice? Ditch the crazy girl. I get she has some serious problems (alcoholic mother, bipolar disorder, self mutilating tendencies, etc) but those problems aren't YOUR problems, so run.. and run fast.

I know you feel like age doesn't matter or whatever.. but seriously dude? She's 13! I wasn't allowed to date until I was 16, & at the time I thought it was "like soooo unfair" but now that I'm older, I realize it was for my own good. At 13 girls are not mature enough to engage in any sort of "meaningful" or "physical" relationship. I'm not sure what your expectations are here. She's already shown you that she's emotional, flighty, and unreliable.. and while that might make her seem like a bad person, really-- she's only showing her age.

So, end this disaster of a "relationship" & find someone your own age.

Find someone who is around your age range. She is 13, she is still very much in middle school. She won't be legal for a few years, just stop <_<