It is definitely a mental thing.I shot pure back tension for about three years to conquer it.If I could provide any advice at all it would be to relax firstly and secondly let the bow know whose boss meaning that just because you draw the bow doesn't mean you have to release an arrow.If you are not relaxed and under perfect control let down.
Professional archers let down all the time,you run the show not your punching index finger.Plenty of articles and books available to help with this,PM me if you would like the titles or authors names.