How can I distinguish between obsessive-compulsive disorder (OCD) and doubt in faith, and what is the way to overcome both?

The difference between doubt (shakk) and whisperings (waswasah) in the context of faith is that whisperings are thoughts that occur to a person, and they dislike them, whereas doubt is the lack of firm conviction about something that has settled in the heart. To get rid of whisperings, please refer to Fatwa No. 103066. As for getting rid of doubt, it is achieved by examining the proofs that necessitate certainty in God's oneness and the truthfulness of His messengers.