Oh I know exactly how you feel! My horse has a very uncomfortable trot and after I learned how to post my riding experience has been made 100% better. Posting is a bit tricky to get at first, but with practice you'll get the hang of it in no time!
I wouldn't go bareback while learning posting just yet, since you are relatively new to the posting trot. Bareback does require more leg though and it can be a great exercise but I would post without stirrups if you want to work with your leg. However, since you're new to posting it's important that you get the feel for it first so you don't end up, as my trainer calls it, "over-posting" and so you don't end up leaning forward onto your horse.
To learn how to post don't worry about diagonals (don't worry about rising as the shoulder of your horse does) just yet. Learn to master the post then work on getting your diagonals right. My trainer told me that in posting imagine an imaginary string that's at your belly button pulling you up and it's more of a forward up and backward movement/ swaying up and back down movement. Don't grip with your knees but your calves, you'll get a feel for this and keep your heels down, toes up don't push your feet forward in the stirrups. Remember to keep knees and thighs soft and lower leg on. When your horse trots feel the bumps, since you mentioned that he has a rough trot this will help big time in posting because the horse will help you rise out of your saddle, push you up in a way. Count to yourself, one two one two one two, rise on one and gently lower yourself down into the saddle on two- stay in this motion. Keep your shoulders relaxed and don't rise/post too high up.
Most importantly remember to stay relaxed and have fun! Don't give up and keep practicing!