I know this may not be of much comfort but , if someone cheats on you then you're better off without them.
Relationships are as much about trust as they are about anything else, when a person cheats it destroys trust, once that happens it's best to cut your losses and move on. In my mind if someone cheats it shows a complete disregard for the feelings of thier partner and if they don't care about your feelings then why should you give a damn about thiers ?

I know it's a cliche but "you deserve better" , that is of course assuming your fears are proven justified.

I hope for your sake that you are wrong.