If you are having a hard time doing one thing -- do the opposite. Let somebody hit you. Really, move aggrsssively into the strike, not to throw -- just to get hit. Get hit a lot. Then you will know where the break point between being hit and not being hit really is.
Exposure lends itself to familiarity, which lends itself to wisdom, which lends itself to reconciling our fears. I think this is a simple technique (as simple as these types of things can be) that one can use early on in one's training - to kick-start things, so as to be able to keep them going, etc.
After exposure, one needs to quest for a truer reconciliation with fear, a more ultimate wisdom - a reconciliation with our ego attachment. Still, in the beginning, best to use your fears - to not see them as obstacles but as guideposts, pointing out the direction you are to head.