One thing to check is saddle fit, with an extra check. Some saddles can sit down on the spine — not under the pommel where most people look, but way back under the gullet where the base of the withers merges with the back.
Easiest way to check in a western saddle is to sit in the saddle and reach into the opening behind the pommel. Have your mare walk around and feel if your fingers are getting pinched.
In an English saddle, you can try looking down the gullet (no pad), or saddling up with a piece of string laid in the gullet, and try to pull it through while you are sitting in the saddle.
My guy went from rushing the canter to calm when I figured that one out. The mysterious little lump that would sometimes appear after a ride also went away for good. This last summer, I had to use a different saddle for Girl Scout camp, because is old one started sitting down on him, and he was cantering with the kids instead of trotting. He is 19.