I have/had what your dealing with. As others have said, it doesnt really go away, you just learn to control it for awhile. It is all mental as you said. I think its the excitement of wanting to see the arrow in the target. I got a tip from one of the masters of the barebow, think volume 3. It is designed to build strength, but it has helped with my snap shooting type TP. I googled and found a on line timer. I set it for 8 seconds. It counts down, 3,2,1, go. I start my draw and am at anchor when it says go. I focus on the target while I keep a solid anchor. Then you hear, 3,2,1,stop. I then loose the arrow. This is teaching me that its good to hold, cause I can tell you I shoot awesome doing this. I get weak fairly soon, so I shoot 2 arrows, retrieve, repeat. The first shot is awesome, second one Im shaking like a leaf, yet 50% of those second shots are right on the money. Im over bowed, hence the reason I tried this technique for building strength. It works well for keeping snap shooting TP at bay. Give it a try, nothing to loose.
Ken