I had similar problems due to two reasons:
I noticed that I would open and close my hand because I was taught the “sponge” technique to half halt when I was a beginner. This has resulted in me keeping my hands too loose and my reins would eventually get longer without noticed.
Also, in my mind, I thought a “loose/open” hand was kinder to the horse because that is how I got the feeling of elastic & soft bit to elbow contact. So I had to re-learn how to have an elastic contact & learn to relax my lower & upper arms while maintaining contact.
I’ll try to put it into words, but I couldn’t describe it accurately.
I had to relearn how to half-halt from my seat. When I want to “close” my hand around the “sponge”, I now think to sit deeper in my seat, but my “seat” doesn’t include just my hips & legs, it includes my upper body too. This requires me to keep my elbows at my side, otherwise, it isn’t as effective for my horse.
To get a visual over the internet
- bring your upper arm so it is parallel with your upper body (perpendicular with the ground).
- have a 90* bend in your elbow
- Part 1) move your right shoulder to right hip backward (as if you were turning your upper body to the right, but not your lower body, other words, twisting your torso). Notice how this all moves together.
- Part 2) while sitting down, sink more weight into your right butt bone (you can be overly dramatic about the amount of weight/sinking, to get the idea). Hopefully, you notice that your entire right side moves down together, not just you hips.
That exercises shows that your seat aids don’t come just from your hips, it is several parts of your body working together. Of course, when I am riding, those aids are much more subtle, but I had to be a bit dramatic in the beginning to train my body (my horse is a saint). Of course, this caused him to react quite a bit when I was half-halting, but now I’m working on “quieter” half halts.
My trainer gave me two visuals that really helped with my problems:
- Dollar bills under my elbows
- Pebbles in my hands
I have stoppers on my reins as constant reminders, I can’t imagine riding without them. Maybe one day I won’t need them.
