If you really need to work on it, it's not there to begin with. It should come naturally. If one of you is bringing up the subject of trust then it sounds like there's already a lack of it.
You say you've been "working on trust". Your gf was in some other dude's car that you don't like. You say you already know that he likes her. Step out of denial and put the pieces of the puzzle together, dude.