I would imagine everyone has struggled with their thought life in one way or another. I heard a minister recently say, "You can't keep a bird from flying into your hair, but you can keep it from building a nest." The meaning being that it is hard to control what you think about for a moment, but you can control whether you dwell on it.

God healed you from asthma and He can help you overcome this. When the thought comes in, take authority over it and change your thoughts. Let this drive you to spending more time with God. Make it a victory rather than a defeat.