Struggling with my romantic relationship

I dated my girlfriend for three and half blissful years until something changed in February and she told me she wanted to move out. We've been living together since August (in an apartment near my school) and since she was moving to a new town she didn't have many friends. The one friend she did have happened to be a sorority girl, so she loved going to fraternity parties as sorority girls tend to do. My girlfriend went to these parties over the months and ended up forming a bit of a romantic relationship with a certain guy.

When she wanted to break up in February I pleaded with her to let me try to change and make things better, and she agreed to stay. Since then things have been up and down and while we aren't technically "dating", we haven't officially ended things (as in told parents/other relatives) and we certainly act like we're a couple when we're together.

Since February she's been doing certain things that I've disapproved of such as trying out opiates and this little fling with the guy from the fraternity, but I've always forgiven her and sometimes things improve; as far as I knew the worst she had done with him is let him suck her boobs.

Here lately we've been really working things and it's almost felt back to normal, but just a few hours ago she left her phone unlocked while she went to the bathroom. I snoop on her phone any chance I get because trust is practically nonexistent right now.

Sure enough she was texting the guy and the most recent text she sent said something along the lines of, "I got my period so we're cool."

I didn't yell or scream or cry. I didn't feel anything. Still don't. It just feels really shitty that she could do that. The worst part is that I thought something like this might happen awhile ago so I made her promise that if she did start fooling around sexually that she would inform me because we've stayed sexually active with each other even though we're supposed to be broken up. Now I'll have to get tested. If I have some disease that'll just be the cherry on top of this shit sundae. I know in a way we aren't dating, but honestly I think actions mean a lot more than whatever we decide to label ourselves.

Anyways, we talked about it and I think oddly enough this may be the thing that finally fixes our relationship. I've wanted it for so long and now I'm not so sure. We'll see how the next few days go. I just hope things get better.