I actually used to have those feelings on a pretty constant basis. Its what led me back to therapy again because it got to the point where it was affecting the relationship pretty negatively. A combination of therapy and communicating with my boyfriend is what has helped me the most. I'm not saying that I don't have my moments where I get worried but it happens way less than it used to.

A lot of what caused it for me is having trust issues in my previous relationships. I had to remind myself that who I'm dating now isn't one of my ex's and that I shouldn't be comparing them. Hope this helps.