If you are sliding forward on the saddle. Then you may have your saddle too far back. Assuming you don't have your saddle tilted very much. Generally saddles should be almost level. But a little down in the front to help with numbness in your genitals/perineal area or saddle sores is okay.

Though I have no idea what your fit on your bike actually looks like and whether other stuff is wrong. You might try moving your saddle forward and your bars forward. Your butt tells you where it wants to be when it's putting power into the cranks. You need to adjust the other stuff to give you a comfortable position where your butt wants to be.