Me [26 F] with my girlfriend [23 F] almost two years together, she still talks to an old FWB [30s/M]. Am I justified in my concerns or do I need to get over it?

My girlfriend had a FWB thing going on right up until she moved to my state. We met a few months after she moved and started dating. This guy, Tim, would send her texts while we were dating along the lines of "You're the one that got away." She would forward them to me and laugh but after a while it made me uncomfortable. She would brush them off but not shut it down. I told her "You need to tell him you're in a relationship now." She did. To Tim's credit, he stopped sending her things like that to my knowledge.

Two years later I am still feeling insecure about Tim. GF got drinks with him once a year ago while she was home for a few weeks and to my knowledge nothing happened. She has given me no reason not to trust her. I haven't heard anything good about him from both GF and her friends. He's apparently arrogant, almost sociopathic in his lack of emotion, likes to push buttons, etc. GF and I recently moved to her home state, Tim is now hitting her up wanting to hang out. However, I feel that I am justified in my concerns about her spending time with him.

I think it's sketchy that he's in his mid thirties and was hooking up with a girl more than ten years younger than him. I don't know him or his motives for wanting to spend time with her now. GF has also started hiding that she's texting him. Last night her phone buzzed twice. I jokingly said she was popular. She laughed but did not immediately check her phone like she usually does. I thought that was unusual so I asked who she was talking to. She said it was Tim and looked as if she was cringing internally, like she either knew she was doing something wrong or knew I'd be upset.

I will own up to my own flaws. I have insecurity/abandonment issues and I was raised by two narcissist parents. GF has said I am trying to manipulate her by asking her not to speak to Tim anymore. I acknowledge that I may very well be manipulating her. This is just something in my gut I feel based on what I know about him and past experiences in similar situations. By spending time with someone who would be trying to hook up with you if you weren't in a relationship, I feel that it's disrespectful to me and our relationship. I've spoken to friends about this issue and they're 50/50. It seems that neither of us are budging on this: I feel disrespected, she feels that I'm manipulating and am insecure.

We had a big fight about it and it seems that we are at a stalemate. I'm not sure what to do. I acknowledge that I may be in the wrong but I also feel that I am compromising my integrity and self-worth by trying to be okay with this situation. Am I justified or do I need to learn to be okay with this?

tl;dr: GF still in contact with an old FWB. Either she's being disrespectful or I'm being manipulative.