It's not the pitch itself, it's the way you've been throwing it. I would suggest that you get together with a really good pitching coach and have him watch you throw a bullpen session so he can see what you may be doing wrong. The sinker in itself is a two-seam fastball, as opposed to a four-seamer,and it shouldn't cause you any problems, so it's something you're doing that may be causing the difficulty.
Let me tell you a story about Jim Brosnan, who was a very good relief pitcher with several teams, including the Cincinnati Reds (and his best pitch was a very nasty slider). One day, when he was with the St.Louis Cardinals, he complained to pitching coach Clyde King that both his fastballs, the four-seamer and the two-seamer, weren't working for him. King called in a catcher and had Brosnan throw for about fifteen minutes, then told him to drop the four-seamer because the two-seamer was working much better for him. Brosnan became a sinker-slider pitcher and had much success with both deliveries for some years. You know, you might do well to pick up and work with a slider, which is much easier on the arm and the shoulder than almost any other pitch and which is absolutely devastating. And you might think about changing your arm slot---your arm angle---and see if that helps. :baseballpitcher: