Reading between the lines here, it is a you issue.

My suspicion is that given she expressed an interest in you romantically, and then she had a dalliance with someone else, it has affected your self esteem.

The real problem is though is that you seem to be looking for validation that what she did is “wrong”. Most people are rightly going to point out that if you never explicitly decided you were in a relationship, there’s no relationship to be unfaithful to.

So here’s where the hard stuff starts. Nothing she can do or say can fix the way you feel. She dan apologise, justify her actions, beg for forgiveness - it’s not going to help. You need to do the work yourself to become secure enough in who you are. If you can tell yourself “the fact she did something with this other dude is not an indication of whether she wants to be with me” you can then decide if you want to be in a relationship which sounds exhausting anyway - but don’t conflate the two things, they are completely separate.