Girlfriend cheated on me... :(

So me and this girl have been going out for a year and a half, everything is good. She is one grade lower then me and all, so she talks to a lot of guys that i don't really know. Well she starts talking to these guys, who have phases of her life. Like this one guy, I'll cal him B. Well B and my gf wuld always talk on the phone, and he would comfort her when we fought (awwwwwwh) but then i check her phone cause she wanted me to reply to a text, well I see that he was calling her a whore and a skank, so I have to stand up for her and i presume to curse him out and tell him to respect a lady. well he threatens to get the "El salvadorian mafia" on me. Do they even have a mafia? But ever since then, she has been trying to get attention from other guys besides me.

Fast foward to yesterday, Were all happy and everything, She got a promotion at her work with a bonus and a raise, so Were happy as more money comes in. I ask her to eat a firecracker with me as i have two. Well She flipped at me saying how I know she doesn't smoke or get high and that I shouldn't offer. So I brush it off, cause I never really asked her.So later that night around 1am, she texts me asking to call her, and I was about to shower so i say after i shower.

When i get out the shower, I call her and no answer, so i text her saying " Am i still calling?" i wait 20 minutes and no reply. then i text again saying " Awh thats so cute, fell asleep after a long night of working sweet dreams" then I'm about to go to sleep 40 minutes later, and i get a call and the first thing i hear is "I'm sooooo sorry, im sorry, im sorry, im sorry." So I'm automatically worried as i think something happened. So i ask her to explain and she's whispering, and She tells me she kissed this kid. Well he kissed her.

Background on this kid( He tries to get with everything that has a vagina, and I warned her and told her not to chill with him after 11pm unless there are other people around, cause he doesn't care if they have a boyfriend or not)

Well back to the story, I ask her to give me a story, and while i was showering, he called her saying to come outside that he's sitting on her steps with gummy bears ( her favorite candy). So she goes outside and they sit and talk (yet she can't see me when i ask) then she tells me that they were messing around with gummy bears and one thing lead to another and he starts kissing her on the cheek then on the lips. So at this point I'm furious and very upset. So I ask to talk to her later so I don't end up flipping out on her and saying something i don't mean. While his is happening she said that she wants to break up(texted me this)

But she's only doing that because whenever i get pissed or upset, She has to try to counter it by doing something that will take the attention from me and put it on her.

Now don't get me wrong, this girl is my best friend, i've been through things with her like no other girl. (Her ex used to rape and beat her daily, while pointing and holding guns and knives to her) So she automatically assumed i was going to cheat, which made me a better boyfriend. Yet, she cheats on me.

So Should I let one kiss get away? Cause it was only a kiss for like a minute, which isn't to bad. Or should i break up with her?

TLDR; Read the story.

 

Just so everyone knows, I'm graduating and Turning 19, and she's going to be a senior at 18.