How can I (18M) run away from someone sensitive (19F)?

So, yeah, six months ago one of my best friends just said that she was liking me, and I was liking her too, so yeah, we got together and all, but since some weeks ago, there's a big problem, she has a best friend, yep, I know, there's a problem, but im not at all the jealous kind and I knew that friend got rejected by her a long time ago, but man, this guy is the most persistent ever, he doesn't give up at all, any girl he tries to hit on, and doesn't matter how many "no" he gets, he keeps going on.

So, I think you guys already understand where this is going, this guys keep flirting with her and she doesn't stop him even though we're together, so yes I want to finish this relationship, but every time that I try to she just creates, and even that she's making my life a mess, I still care about her and doesn't eant to ruin her more than she already is.

I am stuck in this, I can't end things, but I also can't be around a person who calls another guy "soul mate", any help?

TL;DR, I'm stuck in a relationship with a girl with a bad life, she often says that she loves me but keeps another guy around who keeps flirting with her, how can I get away without ruining her?