I've played tennis for a couple years now, and my forehand is more inconsistent than ever due to swinging it incorrectly for all those years. Now I am trying to change my forehand motion, modeling it after my non-dominant arm in the two-hand backhand, which is straight arm at contact.

However, I am having a ridiculously difficult time in modeling my forehand after my two-hand backhand's non-dominant arm. I can swing with a straight arm when I swing slowly, but when I try going faster my arm always bends and takes the form of my old forehand. What can I do to keep the straight arm when swing faster?