Hi, welcome
Ok, so this is an individuals problem. Your partner doesnt actually have a problem.
Trust in a relationship frankly is a gamble we all take whether its a gamble we take with a partner developing into an alcoholic, gambler, secrets involving money or chasing/flirting with other people. Such a gamble takes time often many years to fully trust someone. If you have a previous relationship that has broken trust it is so devastating that the scars dont heal easily. Hence your problem.
The expanse of this problem can come when your partner feels under pressure with his job because many jobs involve communicating with others often strangers. Lets face it, even fully in love with our partners we can feel attraction with other people, then its a case of self discipline and honour to our partners to maintain our boundaries.
I'd like to also suggest that our own thinking is not always open to our partners knowing. We humans are entitled to our own brains and what is stored within, it can seem unreasonable to feel compelled to share every thought. Relationships generally arent that committed so it would be unfair to expect him to share every thought about his views on other people.
This leads me to the topic of jealousy. I had a partner once and we were served by a lady in a store. When we returned to our car I said "that lady has really nice hair". My partner scolded me for it. Then I remarried 12 years ago and my current wife has no fear with such comments and thoughts. I'm a fahsion guru and every night we watch the weather lady on the news and I comment on what she's wearing. "A nice white belt would look better on her" etc. My wife knows I'd never cheat on her and simple admire how women dress, even if a guy is dressed really smart she or I can comment.
IMO opinion counselling would help you enormously.
TonyWK
