Looks like you are doing well, but there's still much to improve. Practice the sitting trot. Relax your legs a bit more and absorb the motion through your back and abdominal. Make sure you keep a deep seat. Don't forget to relax your arms and keep your elbows in. The sitting trot is easier to practice while on a lunge line. That way you can fully concentrate on your balance. Soon you will be on your way to mastering the sitting trot! Personally, I found the sitting trot to be more difficult to learn than the posting trot. Take your time. Good luck!
Yeah, I find the sitting trot in a saddle the hardest out of everything... haha. I can sit fine bareback and on other horse's at the canter. Its just Pluto's right lead canter and the sitting trot that get me every time... =)
1.) Sit back more, there should be a line from your shoulder to your leg. (google horse riding position and you can see pictures of what I'm talking about)
2.) Elbows should be closer to your sides, not out like wings. LOL. Happens a lot with people just starting to canter.
For just starting you're looking pretty good. I a lot of people first starting to canter lean forward, you just have to get used to cantering and develop a better seat. Try posting to the canter, helped me a lot.
Oh, and one more thing stirrup at the ball of your foot, and try not to point your toes out.