Fasting
Our Deen is easy and does not bother you with such trivialities. No sane person licks back its saliva once it comes out of its mouth. The traces of saliva on the lips are licked naturally. This does not invalidate the fast. Had it invalidated fast, no one could ever be able to keep fast, and this issue must have been brought to the Prophet. But there is no such thing in Islamic literature. You are advised to stop addressing your questions about trivialities. This way, you may be able to cure your OCD.