What works for me:
Drop snatches and dead hang snatches - but more drop snatches.
That being said, the problem could stem from lack of core strength, flexibility, etc.
But if its just a mental issue, maybe try adding drop snatches in to a program once every couple weeks, and keep practising full snatches. Dead hang snatches (known to other as tall snatches) are good in limited use to get comfortable dropping under the bar (and force you to stop using your arms to pull).
OH Squats and Power + Squat won't get you doing full squats; you have to get used to hitting that position in one motion from ground to overhead.