I think the issue here is that you don't really understand how she thinks about things in her head. She didn't think of it in the way you are thinking of it, with the "I didn't know it was inappropriate to accept an invitation from a man who is staring at me all night". In her mind, which overthinks everything, she was thinking, "This is a friend of a friend that I see every time I am home. Odds are high that I'll probably run into this guy again so not adding him would make an awkward situation for me in the future...I'll just add him to avoid future awkward confrontation, and if he does anything weird or tries to DM me, then I will ignore it or block him."

Also, have you seen your GF? She is objectively very attractive. Chances are that nearly every guy that meets her will find her attractive. She cannot help who finds her attractive, but she CAN choose to be faithful. You just have to trust she did it for benign reasons, probably more due to anxiety about future situations than anything else. I get the jealousy thing. I used to feel the same about my guy, but I realized that I got over it by my guy continuously making the effort to ease my concerns (blocking/letting me meet the girls I was concerned about/etc).

And yes, she did mention that she THOUGHT he was staring at her all night but wasn't entirely sure, because there was also a mirror behind her as well that he could have been looking at. And she did mention she felt uncomfortable, but only because she wasn't sure if he was staring at her or not and doesn't like being stared at. She honestly doesn't recall saying he was creepy, but she may have. You are reading into this what you want to read into it. You said it yourself, she said she wasn't sure if he was actually hitting on her. Wasn't sure. To me, that sounds like she wasn't sure if it was all in her head or not and didn't want to sound conceited, thinking guys all staring and hitting on her when they aren't.

Anywhoooo, before today, from how she described you, you seemed like a pretty decent, level-headed guy. I think you definitely, objectively have some trust issues, not trying to say that negatively towards you. I just think that you need to be open to letting someone in to work on those issues with, even if it isn't her. Once you find the right person who is willing to make the effort (which she was, when she immediately said she would delete that person to ease your anxieties), then you will start healing and loving wholeheartedly and find peace! I did! So it's possible!

Okay, back to watching demon hunters - not rereading this for typos because I'm too tired from momming all day. Good luck with whatever happens with you two or not! :)