I'm not sure if this is a delusion. I would definitely talk to your therapist.

Do you have abandonment issues?

About a year ago, I had a break down, and part of it was thinking my husband was cheating with a girl on his online video game. I had no reason to think he was cheating, but I was sure I was having a "Psychic" experience and "Caught" him. I hadn't slept in days, and once my stress level went down, I stopped thinking my husband would cheat. I just assumed it was from all my insecurities over being sick. But who knows, maybe it was a mild delusion.

I wish you the best of luck in your relationship. I'm glad you are feeling better and no longer feel you are being cheated on.