What should one do who suffers from obsessive-compulsive disorder that makes him feel as if he has not entered into the prayer after the opening Takbir, and that he intends to leave it after the Takbir?

It is sufficient for the intention of prayer to bring it to mind and specify it, and it does not matter if it is not brought to mind for the rest of the prayer.

Prayer is not invalidated by a lack of devotion within it, and devotion is not a condition for its validity, even if the prayer without it has an incomplete reward.

Doubt about the intention to exit the prayer after the opening Takbir does not invalidate it. One should turn away from whispers (waswas) and pay no attention to them.