Hey, I'm new.
Well... I don't think I can give you any advice on how to deal with this situation, because its honestly something I'm going through myself right now. So I thought I'd just let you know that from what you wrote it seems like we're in a similar boat. I too am in a good, serious relationship with my best friend of 5 years, and the littlest things will always make me worry that "oh, because he wants to hang out with his friends, he must be getting sick of me" or "because he didn't call me on his day trip with a friend to go pick up a new car engine, he must not be thinking of me, which means he doesn't care anymore". But I have the same problem with feeling guilty as if he just pities me when he doesn't do those things. I too wasn't allowed to express emotions, especially negative ones, in my family. The rule still continues to this day. So I naturally have a lot of difficulty with it. But as you said, talking about it can trigger an even more upsetting feeling. And I know how its frustrating to be filled with so many painful feelings, always wondering if he's going to leave because I don't open up, or because when I open up it is all too much for him to take.

So wow... I can really relate. I hope you are able to work through your insecurities, as it seems like you have a good, supportive guy, and once you work through that - well things can only get better.