first of all...sorry for my english..i'm not english mother tongue :) but i hope i will make myself clear :)
Well, i had and have your same problem. with the big difference that the horses i rode were really sensitive to changes in balance of rider...What do i mean? I mean this: if you are not right balanced, your weight on the horse changes easily: sometime you can weight too much on the left side, other time on the right, back or ahead. some horse forgive the rider's errors; some others exaggerate them. my horse exaggerates my mistakes! i didn't understand i was wrong because i feel my body not balanced.....i understood i was wrong because the horse walked too fast, too slow, turned on the left/right instead of going strenght ahead ......
for more than 1 year what i did during every lesson was just to walk!!! i cried a lot of times because the horse didn't walk on the rail :(
the problem was this: i "communicated" to the horse something by my hands or my legs, and something totally different by my balance! I pulled reins for stopping the horse. but my shoulders were ahead of the barycentre and this means "go"...and the horse walked faster instead of stopping!!!
i think your horse is a good guy who forgive you and understand you even though your balance is not correct...but in this way it is more difficult understanding in which way your balance is wrong. this means that you really need an instructor who can correct you!! If you dont have him, ask someone to take a movie of you while you are riding. A movie and a lot of photos :) :)
Go without stirrups, without spurs (i dont know if you use it usually), and without reins (it means that some1 has to hold the horse, of course!) and try moving (=communicating) the horse just by your balance. when you will do this walking, you can try trotting :)
It is not easy to do :)
bye and sorry me again for my mistakes in english :( tell me if you have understood what i meant