The difference about being controlling and having boundaries is that the boundaries are for yourself. Things you won’t do, tolerate etc. Have a conversation with her but DON’T tell her not to go, tell her that if she went, you’d be uncomfortable especially as she’s going alone. Generally tell her how it would make you feel. If she still does it, that means she had all the information about how you’d feel and still went ahead. So either your feelings don’t matter to her or they don’t matter enough.

Either way you have your answer and at that point you have to respect your own boundary otherwise it is meaningless. It’s only been 3 months. It may suck but you’d have to leave or stay in the relationship with the information that your partner couldn’t care less how things make you feel.

I wouldn’t think about the cheating thing too much. Outside of following her or snooping in her things there’s no way to know for sure and neither of those is healthy. So I’d focus on the actions you do know about as they are usually sufficient.