What is the ruling on one who experiences obsessive thoughts during prayer, over-prepares for it, interrupts it for the slightest mistake, and is unable to swallow his saliva during prayer? Is his prayer valid?

You are not required to swallow your saliva or spit before prayer because swallowing normal saliva does not invalidate prayer or fasting. Intend the prayer in your heart and do not vocalize the intention. Say the takbirat al-ihram (opening takbir) and do not repeat it if Satan tempts you with whispers. Do not obey the whispers in any action of the prayer you perform, because your disregard for them leads to the despair of Satan. Do not interrupt your prayer for any mistake, because interrupting prayer without a valid excuse is forbidden.