Feelings, memories, fears are encoded via neuro tracts in the brain. If the neuro pattern is laid down during the "fight or flight" response, the chemicals present at that time in the brain make the neuro tract very strong and may take more work/time to change. There are a number of methods that will over write those patterns and or disconnect them. Most phobias are easy and fast to cure in one to two sessions.
While phobias are included in the manual that is used to diagnose mental conditions, I would not put them in the category of a "mental illness" but more condition that any of us can have and many people do. What is not known by most is that the more one talks about the phobia or event that sets a neuro pattern the stronger it becomes. Turns out that our grandmothers who told us to forget it and move on were right.