At least for lessons that is. I don't have my own horse so I ride lesson horses. We're only allowed to jump a maximum height of 2'6. It frustrates me that I won't get to experience jumping higher. Unless I get my own horse.
I don't know if this helps or not but maybe you can look at this as a way to have the best Eq possible. I mean you'll have an advantage(SP) over someone who was rushed through their jumping heights. You're eq over anything 2'6" and under should be killer! I don't have any restrictions other than the horses I ride. Zilly does about 5' easy but I've taken him about 3'-3'3" for now because I feel like my Eq falls apart at the height so I wont go heigher till my eq is darn near perfect :P
Could it be an issue of protecting the horses from being overworked??
I know 2'6 may not seem much, but even if (hypothetically) the horse is doing just 2 lessons per day every second day at that height, it is still a big ask for him.
They have to do what is best for the horses. It may be holding you back, but if they were to let you progress, then they would have to allow the other students as well. They have to draw a line somewhere for the well being of the horses.
I once had an instructor who would put up the jumps higher without warning, and she would push me to do all kinds of scary jumps that I wasn't ready for. I mean, I was physically ready, because I did jump up to 2'9'' with that instructor, but I was not mentally ready in any way. Eventually it got to the point where getting over the jumps was more of a relief than it was fun.
Now I ride at a place where the instructor has only been setting the jumps up to about 2'3'' - 2'6''. To be honest, I'd rather jump up to 2'6'' and have fun than be rushed into jumping any higher.
I'm not saying you won't have fun jumping higher. But think of it this way (this is how I think of it, so it might help):
The height of the jumps does not determine how good of a rider you are. And just because you are not jumping progressively higher in your lessons does not mean you aren't advancing. There are so many more ways to advance besides jumping higher. In my lessons, the height stays the same, but we learn to do things like courses and grids. The jumps are set up differently each lesson and it is always challenging and exciting.
A person who can jump a 3' vertical in the center of the ring may not be able to get around a full course of 2'6'' jumps, and vice versa. Both riders are advancing, but if you have a jumping limit, you will just have to keep learning and improving in other parts of your riding, by challenging yourself and working on bad habits, etc.
I've been with my trainer for 10 years now and she tells me to take my time with height. Beacuse I train and exercise her horses, she trusts me that I wont go from 2'6" to 3'9" on them. They are mainly ponies and horses that are very green, and need the jumping background. I'm one of the older girls at the farm but i'm the only one who really jumps. The younger girls enjoy it, but my trainer knows my potential and so she doesnt push me to do anything I don't want to do. . .but she pushes me to succeed to my potential.
But I understand that since you are riding lesson horses, you can't push them since they are being ridden by many other students. It CAN tire them out and its hard to have an active lesson pony for so many students when they are burnt out.. . .
I ride lesson horses too as I am not yet allowed one of my own though my riding instructor says that I am sooo ready for one !!! .. anyways ... I ride a lesson horse called oscar and he is so great and everything but ... the jumping height restriction .. it mite be because he can't jump relly high but I think the highest I've jumped him is about 85cm .. and with the lesson horse I used to ride (gemma) ... she was aan awesome jumper (she got sold tho :(..) but I jumped her over a metre and stuff and im tired of not being able to jump high when I know I can :(