Hi, I ride both western and english, but started english, and it's still my main discipline. I also ride hunter/jumpers and have ridden them for about 4 years. You probably loose your stirrups mainly at the posting trot, right? That is because you use your stirrups to post, and it's an annoying habit, but can be fixed easily. One thing you can do about your stirrups is simply drop them. Yes, take your feet out and cross them over the saddle. Get a friend or trainer to put you on a lunge line and just do posting trot for about 5 minutes each direction, and build from there (example-7 minutes each way next time, then 10 minutes, then 13, and so on...) It will hurt like crazy for a while, but eventually you will be able post with ease without stirrups, which is sometimes required in some english classes at shows. After you go 5 minutes each way w/out stirrups, pick them up and continue posting. It should feel easier. The point is to teach you to not depend on stirrups. It will also help you keep a steady leg and independent seat, which are both GREAT things no matter what discipline. Other notes/tips: have light contact on both reins, so you can "feel" the horse's mouth. Most hunter/jumpers want
contact, so they know what you are asking. Also, remember to always hold with your outside rein when bending, don't just "drop" it, or take the pressure off when turning, or else the horse's shoulder will "drop." Remember to sit up tall and breathe, and keep a slight bend in your elbows. You want everything to be in rhythm with the horse. And your two-point, or jumping position, don't ever stand in your stirrups while going over a jump, it is not the correct way, and so many people do it. You simple reach forward with your hands to give the horse his head and tip forward from your hips. Hope this helps! Good luck!