One leg posting is VERY hard, so don't knock yourself there. Ironically posting without both stirrups is easier, and cantering without them is easier still. Just keep trying until you build up the necessary strength. If you're lucky enough to have a horse who will humor you by slowing to a walk whenever he feels you giving up, just do a few trot steps at a time and come back to walk to reorganize. Until you get the strength to do it just from your legs, keep your reins long and hold the pommel with your hands to help get you up out of the saddle.
You just have to see your distance...you don't have to like it.