Is saying "I will not perform ablution," then [later] "I will perform ablution," considered disbelief if the person suffers from obsessive-compulsive disorder, and does he exit Islam because of that?

What you said does not affect the validity of your faith, and there is nothing wrong with it. You must turn away from obsessive thoughts, strive against them, endeavor to get rid of them, and beware of being carried away by them.