Looks like you have the saddle all the way back in the clamp. Move the clamp more to the center of the rails and as another person said, make it level with no tilt going down. If your saddle has a rise in the rear of it, then ignore it for leveling purposes.

Usually sliding is more about you being told where your butt should be positioned over the BB as your exert force on the pedals. There are some other things to consider, but you need to find that position first and then work out your other issues afterward. Though you first need to find out if you even have the correct saddle height, which is pretty much numero uno for you to determine.