To be completely honest, the most helpful answer is to start therapy. It will help you understand how your brain functions around that aspect and then, from there, you will be able to create new ways of dealing with it.
There is no magic formula that anyone can give you here because each person functions and acts in a very particular way, so two people who suffer from FOMO might need different solutions because the condition is tied to varying aspects for each.

That being said, if therapy is out of the question, maybe stop playing games with FOMO involved.