Unable to digest girlfriend's past. Need elderly advice.

Hi gentlemen. I'm young, 19M. in a relationship with a girl(20F) for over a year now. In the beginning everything was good. Still is, but the more i learned about her past the more it is creeping to me.

To be short, in her last relationship, she called the guy to her house(She lived with her parents that time), the folks caught them together, Her dad disappointed in her fixed a marriage for them. She agreed to it and was looking out very eagerly for it. Unfortunately, the guy's family denied for some reason and that's how they broke up. They named their future kids and what not. I feel...bad after knowing all this.

Also, i have quite a history with this girl, In HS I Proposed to her and she rejected me and made fun of me for thinking she'd choose me and we would have a future together; told me she was not ready for a relationship...Yet, a week later gets in relationship with another guy, writes porn about him, fucks him and what not. The same guy she wanted to marry.

She has apologized for all this, all of what she said and done to me in the past and i did forgive her for it. She has changed, really. But you know, it is creeping back up to me again. The trust issues, the disrespect, the lies(Not ready for a relationship yet), Her wanting to marry someone else; I keep circling around the fact that the only reason she is with me is because the guy's parents called off the marriage.

I'm quite fucked. In college right now, Mechanical Engineering. Grades were good last semester, This semester they have dropped over this mental fuckery.

Any advice/opinion is appreciated.

Thanks guys.