It takes lots and lots of practice to get the strides right to the jump. My daughter has been jumping for years and she still has to work on that. Especially when she's on a new horse. Each horse is a little different. Just this week, her trainer had them all in a flat lesson going over two poles on the ground. At first she had them do it in four strides. Then they had to adjust their speed to make it work in three strides. Trying things like that should help you learn how to get your horse to where he needs to be for the jump. Also, if he's running at the jumps, half halt him several strides before the jump. Giving him his release over the jump is exactly what you want to do, so it sounds like you're doing the right thing there. If he speeds up after the jump, then half halt him again right after the jump.
One last thing, definitely find a helmet somehow. I've heard too many horror stories. You only have one head and one brain, and it's important to protect them! ;)