Does she let you jump higher with other horses, or just your horse? Theres a lot more to jumping then just scope, at higher levels the horse needs to adjustable, straight when riding to the jump, and carrying herself well. Your horse could be physically capable of jumping that high, but maybe she's not quite ready for that yet. I would ask your trainer why she hasn't had you move up to bigger jumps. She could have a reason for keeping you and your horse under 3'.
First thing is always to ASK YOUR TRAINER. We don't know you, or your trainer, or your horse. There could be a good reason, there could be a bad reason, or there could be no reason. If you don't have the kind of relationship with your trainer that you can ask such questions, THAT is a red flag right there, although who is to blame for that is not knowable from here either.
Leaving a trainer because you want to jump higher could well mean you find a trainer who doesn't care much whether you are ready or your horse is, but will take your money regardless.
A lot of horses and riders never go higher than what you're jumping right now. Especially in hunters. You can do a lot with that height. Jumping higher jumps doesn't mean you're doing "better" as a rider, and it's more wear and tear on your horse.
If she's bucking and spooking at fences and when you're riding a course, your goal should be to get her more confident and moving better instead of jumping higher. It's one thing to have a bit of a rough ride over 2'3", it's another matter to have a horse bucking and spooking over a 3' course. The higher the fences are, the smaller margin there is for mistakes and rider error, and it could become dangerous to you and your pony. There's no shame in not jumping higher, and you could always ask your trainer if you could jump higher on a different horse if you really wanted to. Ask your trainer if she thinks you and your pony are capable of one day jumping higher and ask her what she thinks you need to work on with your pony in order to jump higher, and make that your goal. I think your trainer is looking out for your best interests.
I would talk to your trainer about it. Be honest with her about your goals, and if she thinks they are realistic for you and your pony, ask her what you need to work on to get there. She probably has a reason for not moving you up higher, and remember that jumping 2'6" is already a great accomplishment! Since she's recovering from colic surgery right now anyways, I wouldn't worry too much about jumping.
Make sure you check with your vet that continuing to jump will be okay for the horse. In some types of colic surgery, the intestine is attached to the abdominal wall to prevent twisting, and that can make jumping unsafe for the horse. I've ridden a horse that couldn't be jumped for that exact reason. So just make sure you get the all clear!!