I don't know that much about tennis yet, but from my experience in table tennis, I can make a couple of guesses:
Because you're nervous when hitting the shot, you tighten your muscles more then you usually do, so you might need to relax (your arm) more.
Maybe a solution is hitting the big forehand a lot during less important points, when you're a bit more relaxed. If it starts to work like it usually does, just keep hitting it untill you have the confidence tot do it on any point.
Perhaps it's not the same in tennis, but if it is, I hope this helps