What to do..., when you don't know if you can trust someone or not? Options

[Tsukuyomi-No-Mok...]

So me and this girl we've been on and off wit each other for like a year and some.
When i met her we got along real well and found out we both from CT
now we got real close and then got together then my ex came into the picture and screwed everything up cause she ain't like the girl even though i didn't wanna have anything to do wit ma ex. because of this the girl i was wit got mad at me cause of the lies and shit people told her and she then didn't trust me cause even this one kid i was cool wit went behind ma back and screwed everything up.
So for the rest of the semester i worked at fixin the stuff up
and over the summer we got back together.
this time we got into an argument cause throughout the time where i was tryna fix things up wit her she basically blew me off no matter what i did for her so when i left the school when school closed for the summer i flew up to PA and NY and stuff and met this other girl kicked it wit her for a lil bit and then basically called her ma girl even though it was a bounce back chick nothin real serious
so over the summer near the end of it she got madd bout it cause i never told her bout the girl even though ther was nothin serious goin on between me ad her ne way.
so when the first semester in the fall started she was madd at me and i managed to explain everything ot her and we began workin on things all over again.
so we've been talkin since then and she wanted to get back together but i told her to wai tand let us get to know each othe rbetter so we could handle problems wit each other better.
and she got up set but also cause i had to handle my own life seein as i was already in danger of leaving the school adn goin to a completly diferent state. but we remained talking.....

now i'm here this year and i find out from my frat brothers that they've al been smashin her but they didn't know that we were talkin or of our history since she went and hooked up wit brothers from other chapters.
my chapter brothers however infomed me cause they knew my relationship with her whereas the other brothers didn't know and then wen i mentioned her they thought i was jus tryna smash her for the hell of it then when they found out i find out that while we've been talkin she's been hookin up with them and all that.
however she only told me bout one of them.
So now I don't know what i'm supposed to do cause now she's acting funny round me and she doesn't really say much to me and all of a sudden she randomly jus goes out like at random hours like 1,2 in the morning and all that and then comes back and jsut acts funny towards me yet everyone's tellin me how she misses me and all that.

So i'm lost cause i don't wanna throw all the stuff that i know bout her in her face one cause my frat brothers i dun wanna put them on blast and i'm usually around them alot
whereas if i'm in a meeting it's madd akward cause when i see them i thinkof all the times i've had sex wit her and then i'm like damn it's like i had sex wit them too.
iono why she's not really tellin me but i'm assumin she don't think i'll find out.

Now i mean everyone tells me to forget her and all that and jus move on but for me that's kinda tough seeing as i realy care bout this girl and all that
but iono what to do bout this whole thing cause more and more things lookin alot more shady to me and iono what to do cause iono whats going on.

so my question is what do ya do?