I think you mistakenly posted this in the health and fitness forum.
It sounds like you are going to have to rely on your forehand to change directional patterns if you continue to have this problem.
The single biggest tip I can give is to set in your mind that your contact point is a little further back than the DTL shot. While you can hit it out of an open stance, try working on a more closed stance. Get your racquet back fully with good trunk rotation, and on the forward swing keep that forward shoulder down as you lean into that shot down the line in a forward direction with your whole body at contact and in your followthrough.
But to better your DTL backhand you likely need a coach to feed you balls, a hitting partner to work on DTL backhands, or use of a ball machine. You've got to reset your timing for the new contact point, getting in the right position, and maintaining that forward momentum down the line with your whole body. That takes hundreds of balls to get the right timing down and make it consistantly reproducible. And because the net is higher on down the line shots and the distance shorter for a long ball than crosscourt, the DTL will always be a lower percentage for anybody, not just you.
Good luck!
