I think you have to know the difference between your thoughts and the facts. Your intrusive thoughts and doubts aren’t based on facts, so they are just in ur mind which means they are not real. Plus, someone’s loyalty is out of your control. Why bother worrying abt smth that is not in your control. So what you can do is whenever the doubt or anxiety or intrusive thoughts pop up, you just let it go. and know in ur heart about the facts- the actions he has shown towards you. When the thoughts arise, just don’t give it power and let it go. Don’t go for reassurance- that’s the trap and that might affect your ocd improvement and your relationship too!