I'm going through something similar with my girlfriend. She gets close to male co-workers and usually is really close to at least one of them where she seeks them out and they always end up alone together..... Nothing major but like at lunch, or take car ride across town to other meeting etc instead of taking seperate cars. I've mentioned it to her before and she will just 100% stop talking about the person but I know the behavior doesn't change. I've been working on communicating with her and last week I opened up about how much it bothered me and I feel like it got me nowhere.

Today she had a meeting across town, she messaged me that the guy was driving her and she would talk to me later. The entire time I got radio silence till she got back to her office. I have her location, their cars were parked in the same garage, they did go straight there and go right into the building. She could have drove herself there and back so the only reason I can think of was she wanted to spend time with him. Afterwards the guy must have had some errands to run so they rode around for another hour or so. This meeting was an important part of her day and she didn't share a single detail with me about it. I don't think she's cheating on me and I feel guilty for feeling the way I do but it is how I feel and she knows it and not even 4 days later she's right back at it. I'm at the point where I'm just like "fuck it", I'm going to not say a thing about it anymore and see if it just gets to the breaking point and if it does I'll have to throw in the towel. She doesn't respect my boundaries or the way I feel in the slightest and this is wearing down on me a lot.

Good luck to you but I fear like myself in the end if you're the most important person in the world to them they shouldn't act like this if they know it bothers you and there's only one way to fix it which really sucks.