Hi - I think until you feel more comfortable in your leg and get more consistently balanced over the fences that you should be doing a crest release. Once you can do a crest release without any mane and keep your leg under you, then you can start to transition to all auto.
I have to admit that I am an individual who learned how to jump by just doing it. So I don't know what your instructor means by saying "perching." I would think that she is saying you are tipping forward through your hip bones instead of folding in the hip. If this is the case you have to come to the understanding (first on the flat, then over fences) that your spine and hip bones are all one solid straight apparatus (for lack of better word). You should have a strong, straight waist.
First, do you know where your hip joint is? If you sit and move your leg 90* out you will feel an indent where the leg is held into the hip socket by muscle - follow that indent in and there's your hip.
The top of your hip bone is your waist.
When you are jumping, your waist should stay solid and attached to your tailbone. Your entire spine should stretch, your head forward, your tailbone backward - your back should feel flat and wide.
When your horse comes up under you, propelling himself upwards and forwards, your ankles, should become soft so that they flex making your heel drop that closes the angle at the front of the ankle. Then your knee angle opens slightly which closes your hip angle. Your buttocks don't need to leave the saddle but half an inch - and the amount will depend on your horse's jump - you should allow the horse to move you. There is no timing involved in the grand effort of hip folding - simply relaxation that allows you to follow and stay in balance with the horse's motion.
Again, I love a line of bounces (or one strides) to get my seat correct over fences. It's the ultimate 2 pt exercise
When all else fails - it is far better for your horse and far safer for you to be in the "back seat" slightly left behind rather than falling forward.
But if I were you, I would sit down and have this discussion with your trainer - don't have it from on top your horse, do it actually face to face, on the ground. Express your concern that your position is becoming weaker - and give her time to respond, it may be that she has a completely different thought on what is a correct jump than you do - and this is what you need to talk to her about. Ask questions during your lesson as in "Is this where you want my leg over the jump" and position your leg - make it obvious when you are confused and need help - that is how you glean the most from every experience. I always am happy when something goes wrong in a lesson - because it's an opportunity for my trainer to help me troubleshoot a real life situation that may occur every day in my real life! OK done with my philosophy on life... :)