I know your pain! That is a tricky process. The worst part for me was that damn trigger assembly wouldn't stay.
But a trick is to hold together the front part of your gear box first, then when that's tight slowly work the other parts together, it helps taking breaks, because as you know its very frustrating, and rushing anything in the gearbox will only make things worse. Make sure everything is in the right place; facing the right way before you try to put the second half on.
Is it a jg btw?