Many moons ago, when I asked the major league pitcher who would become my pitching coach about the slider, he told me: "Throw it like a curve, but roll your wrist, don't snap it." He showed me the off-center grip he used---and believe me, it was off-center, neither two-seam nor four-seam but in between, with the index and middle fingers close together---and demonstrated the wrist action, which is a lot easier than what one does when throwing a curve ball. It's quite possible that you're throwing it too hard, so what you want to do is ease up on it---just turn it over. And a lower arm angle just might help; I was a natural sidearmer and threw everything that way, and I had no trouble at all with the slider. He handed me the ball and said, "Go ahead, try it," and in about ten minutes I got the idea. I worked on it over the winter (1951-52), and it became my strikeout pitch.
So give that a try, and good luck. 8)