I used to work with a Theraputic Horseback riding place and we basically did a reverse of the mounting. Gather your reins, take feet out of the stirrups then lean not so much forward but forward and to the side. Use the pommel or neck for support. You may have to use your left stirrup for balance and support at first but be sure once you have your right leg over to balance over the horse and the saddle (even lean onto the horse and the saddle) and drop that stirrup before you try to slide to the ground.
Practice makes perfect. Just keep trying. Break it down into smaller steps and I think you will find a way to make it work.