What helped me with my elbows (only just yesterday at my lesson, ha ha), is to bring them back so that they're pretty much resting at your hips. That's where they should be. It will feel weird at first but I noticed a change almost instantly in the way my horse was going and carrying himself. That being said, he goes as an exact mirror of his rider, and is great for teaching you where you should be. I also do think that your leg is a little too far back. Really sink into your heels and use them to anchor you in your tack. Maybe stand in your stirrups for a second and let your leg get nice and long and your heel come down. Than focus on keeping your leg an heel that way.
I give myself very good advice, But I very seldom follow it