first of all, I feel bad that you feel necessary to say "Scared to post this", I suppose because you fear being roughly treated? I sincerely hope people aren't scared to post videos for critique.
I am glad you overcame that fear and posted. Good on ya!
Ok, yes your arms are incredibly stiff and straight. However, if you were to magically photoshop them off your body (painlessly) and watch the clip just pretending you have NO arms at all, then your body and posting is not half bad! seriously, look at it again and use your imagination to erase your arms, totally, so you are just a torso and legs. the rhythm is steady, the leg is quiet, the angle is decent.
So, this problem of your arms is what you must change. And the thing is, when you change this, everything else will fall into line more than you can guess. Because the way you hold your arms affects the connect to the bit, which affects your horse's willingness to trust any connection and to put any real Umph and drive forward with confidence into the connection.
It's more than just looking right.
you need to get your upper arm connected to your rib cage, put a bend in the elbow, roll your shoulders back , get the thumbs on top, and hold the reins so that your thumbs point like LASERS straight from your elbow, through the thumb and to the bit rings.
This position will give you so much more strength and solidity, and you'll be better able to ask you horse to step up and through his back and bring the energy forward to the bit. the more energy he offers, the easier it is to post, and the easier it is to keep that firm upper body, and it's round the circle again.
sit on your horse reins loose, arms hanging down by your side. now roll your palm as far outward as you can, so much that you feel the arm roll outward in the shoulder socket. hold that . now, holding that position of your upper arm, bring the whole lower arm up into "holding the reins" position. Do not allow your elbow to leave your ribcage. THAT is how it feels to hold the reins in a good position. You can allow your elbow to open and close its' angle, and move forward and backward a tiny bit, but your shoulders are rolled back (imagine you have a million dollar bill tucked under your armpit, toward the back part of it), and your elbow always has some bend, thumbs on top.