Good job on the progress! I've been exactly the same way as you, and I know how hard that is.

As for the dream, dreaming of your partner cheating on you is most commonly related to insecurities, fears, low self-esteem etc.
I wouldn't pay much attention to it, obviously you are still insecure and afraid on some level - but as long as you manage to control it when you're awake you should be fine. Your mind runs wild when you sleep and process all your struggles and subconscious battles, so it's only natural that it would include that.

I've had dreams (nightmares) of my bf being unfaithful as well, and I know how that can trigger the paranoia - so just do your best on saying "It's just a dream" and go on with your day.