My girlfriend met a man online and now she wants to go to a convention alone with him, can I forbid her from this?
Why don't you want her to do that?
I came home early one day and he was already in her room lying on her bed
Wow, that seems weird. I think it's perfectly normal not to want your girlfriend to go somewhere with this strange guy.
She's not a virgin. She's cheated on me twice before. I was stupid to think she would be faithful.
If she has already cheated on you so many times and wants to go to a convention with another man, then you need to speak with her about this. You should have the upper hand here, and she should be respectful of how you feel.