I actually really like your position! You have good posture! But your legs need to be just a little more forward. I like to keep my feet by my girth because I used to have my legs too far behind me and it would throw me off balance. So really push your heels down (your heels aren't bad BTW) and push your leg a little more forward, but don't over exaggerate(:
For your diagonal, sit trot. Get someone to sit out there with you who knows diagonals. Sit the trot and close your eyes. Start trying to feel for the diagonal. Once you think you feel it, post. Get the person to tell you whether your on the correct diagonal or not, and just keep practicing both ways. Once you get this down, getting your diagonals will be second nature! Also, when you begin the trot, you don't need to post right away. Sit the trot a few steps; if you have to, look down while your sitting, then pick up the correct diagonal. It doesn't need to be rushed, just allow yourself to check before you pick up the wrong one. My friend used to have the same problem!
When you canter, sit back. It looks like a bad habit of leaning forward to almost "help" your horse to get the lead. Sit up straight like you do at the trot, if you need to tighten your inside reign so you can see the corner of his eye, and apply pressure with your outside leg. Keep practicing! It will all resolve in no time!
Generally, your eq is pretty good. Good posture, your already dropping your stirrups which is AWESOME, and you have good hands, elbows and heels. Like I have said before, start waiting for the diagonal to come to you and start feeling for it, and bring your leg a little more forward. I really like your horse actually, I think he's cute! And with hard work and practice, you'll do great in the jumper arena! Eq might be a little to quiet for your horse, but then again, anything can happen! I bet he will be a really nice guy in no time(:
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