She doesn't treat me like crap.

OP, she went behind your back and was cheating on you. Then, instead of coming completely clean, she continued lying to you about details.

we played a lot of League and maybe my attitude is what drove her to do this.

Just STOP. You can't make someone else cheat. Your gf made the conscious decision to go behind your back, start texting other people, then sexting, breaking it off, etc.

I was pretty competitive and a bit aggressive at times, but never went overboard.

You know what decent people do if they don't like something? They sit their SO down and COMMUNICATE. If there's no compromise, they end it. Cheating and lying is NOT what decent people do.