1st Vid: You seem to get really jostled at the canter. How often do you get to ride? I would try running to help build your muscle in your leg to help sit more. You are behind the motion, try pushing your butt back a little more and arching your back. I also noticed that your hands have a tendency to move around a lot. I would try and keep them much stiller (is that even a word???) so you dont yank on your horse's mouth. When jumping you were very crooked. I'd go back to a single jump and focus on riding it totally straight. This will create many problems if not fixed but flat work should help.
2nd Vid: This video was much better. I liked that you stayed in jumping position the entire time, although your leg was still swinging. The steering issue was almost gone, until after you landed and the horse tried to cut in. However, you worked on fixing it. The one stride was very nice but the 3rd jump was a little "tight" meaning you got there two soon.
3rd Vid: Your horse is racing to the jump as you mentioned and you left to far away. As you mentioned your back is hunched - most likely because you are just trying to stay with him. As for the flat portion, i see many of the same faults as the 1st vid.
0:39 your foot is too far into the stirrup and you need to stretch up. 0:48 - your hand is still moving too much and i would lower your hand more. You seem to be jostled around again.
0:58 - i know you couldn't do anything but the girl rode up behind you and your horse almost kicked out. This is very scary and unsafe. I'm sure it wasn't a great feeling riding around right then!
For the course: After the 1st and 4th line your horse doesn't get its lead change right away and your counter canter through most of the corner, which is scary because you can potentially fall down. At the 2nd line your reins get way too long so your horse begins to canter faster, and faster and faster.
I would suggest holding of jumping until you can get your flat work a little more polished. Do you own this horse or do you just take lessons?