For the striding, I think it just comes with time and experience. When I first started jumping, I would LITERALLY get in position 3 strides out. I got my new horse who has a gigantic stride, and the more I rode and jumped him, the more I was able to see my spots. Now, on any horse I can get on, I can jump a couple jumps and am able to see the spot regardless of the stride of the horse. Everything, including your position will come to you in time.
Now what I did when I first started jumping, was ALWAYS use poles. Always always always. If it works for you as you have stated, USE IT. As my trainer says, you have to prepare for the "5k". If you don't prepare, you will never achieve the success you desire. Also, set up the bounces as one of the posts had said before. It will help you with your position AND your spots. Also, having a strong support (your legs) will help with the standing up. Actualy, a strong support helps with everything you do riding wise.
For your position, take your trainers advice. Allow the horse to come to you. Whenever I jump, I stay still until I know my horse has jumped, and I allow his neck to come up to me. I hate to say this, but even if you have to grab some mane, do so.
Sooner than you know, you will be able to see your spots and will be able to adjust as needed. You will also have a fabulous position! Like I said, it comes with lots of time and preparation! It can't just happen if you don't put forth the effort.
Goodluck, and it will all happen soon!
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