Hi, I’m really sorry you’re going through this. Sadly - this is a really common delusion that we see on these forums.
Here are some examples from the past (just so you understand how common it is):
Here: My boyfriend thinks I am cheating on him
Here: My husband has severe paranoia and thinks I'm cheating and trying to kill him!
Here: Constantly accused of cheating. Controlling husband
Here: Accusations vs. Reality - How to Deal with "cheating" accusations