If you have trouble getting your legs back and underneath you need to stretch your hamstrings. This is easy and should make you laugh. Get a piece of 2"x6" board, maybe a foot long, doesn't need to be longer. Put on shoes or boots with a hard sole and stand on the board with the ball of you foot, place your heels on the floor. Do this near a wall or big chair to help with balance. Your bum is sticking out and your knees are bent, right? Hold onto the wall or chair and straighten your knees. OK now pull you bum forward and stand straight. You will feel it pull from your heels up into the glutes. Hold for a few seconds and bend at the hips a little to release. Do this about 10 times daily to start. Watch tv while doing it. Do it daily until you aren't feeling much pull. This will enable to you bring your legs back in to a better position in the saddle. It might take a month or more but it does work. Been there. To prevent slouching, when seated in the saddle, pull both arms behind your back and pulling your shoulders back. At the same time, try to make your belly button touch your spine by pulling it in and lifting your chest. When you get these two issues resolved the rein handling will get better.