1. He needs about 100 pounds. Feed him, hehe.
2. He is not collected at all... he is just working off of the face. His back didn't move... Look at the clips where you change to neck reining--see how he just sticks his neck and nose in the air? This is what is causing the bad topline; riding him in that manner. If he sticks is nose out, he's not carrying you at all. But mainly, look at his back when you fix his head--there is no change in his back at all.
3. There is absolutely no impulsion. Yes he was tired, but slow and right is harder then impulsion and right! Which means... you were going 'slow and wrong' :(
4. He looks like a whole different horse in his older videos. He is more rounded because he has much more impulsion! You need to ride him at that kind of gait for a while... but don't be surprised if he can't handle it for long--he needs more weight.
5. Please consider getting his feet checked... he moves so much better in his old videos and I think part of it is because his hoof angles look so off.
6. We are all saying 'work through the bit!' So when you are pushing him forwards, with impulsion, think of wanting him to reach down into that bit, with all of his energy going in the direction of travel--so you don't want to stop that energy by creating a 'wall' with your hands! Be soft and inviting.
Remember, collection doesn't come in a day. We don't get out our horses and think, "I want to get him to collect--there he goes! Perfect!" It is an on-going battle, something were your horse gets better and better. You're not going to ride him one day and just get him to magically lift his back because you move your hands differently--it takes muscle, and he physically just can't do it, he's not strong enough. So each day you work on lateral work and long and low and play in the gaits... and he will eventually get to a point where he can go, 'hey... I can carry her!' There are levels of collection, it is not a definite thing.