The lunging before mounting needs to be done in a way that encourages the horse to relax his muscles, not just warm them up. Which means no trotting until the walk is completely relaxed. No cantering until the trot is relaxed and free. Otherwise, you are allowing this cold backed horse to practice going around with tight, restricting muscles. Instead, have him practice going soft and quiet. Transitions help relax. A trillion of them.
Another good thing to do is stand beside him, and grab the front of the saddle and gently push the saddle away from you. Keep pushing until he stabilizes his feet. Then pull the saddle towards you. Keep pulling until he stabilizes. Push and pull slowly and without being abrupt. Do that until you can push and pull the saddle with him only rocking his weight back and forth and not moving his feet.
Make sure you are using a mounting block, or a fence, or something. If your horse rears, then you need to abort mounting and quietly ask him forward. Take him for a walk. Return back to the mounting block. Quiet repetition will help.
Maybe mount her in a totally different area than she's used to.
Break it down into small steps. If she reacts to any of those small steps, don't even think of continuing on. Make sure she stands quietly for each step.
It really isn't just about warming her up and hopping on. To really help this horse you have to take your time and respect the fact that humans have given her an unpleasant mounting experience and its your job to correctly and sympathetically fix it. (Not saying you caused this problem)
When you're riding, same concepts apply. Don't trot until her walk is relaxed and smooth. Don't canter until her trot is relaxed and smooth. No reason for her to practice being tight and un relaxed when there are ways to make the whole ride a happy environment.
Sorry so long. Heh heh