If your mare is jumping 3' over a pole, it sounds like she is in as much need of form help as you are. Gymnastic grids are your friend for both of you. Nice easy pre-determined striding for her and the ability for you to just sit there and wait for her to jump. Start with something very basic. Four trot poles spaced 4'6" and then 9' to a cross rail. Set it up in the middle of you ring. Trot in over the poles and then just stay in the up beat of your post (half seat) close your leg (bear hug your horse) and plant your hands about 1/2 way up your horse's neck to provide a release. She will most likely canter out. Come back to trot, and come back around to the grid. Change which way you turn each time after the cross rail.
If your quick mare still runs out, first make sure you are using enough leg and providing her a release. If you are and she is running out, use wider reins to channel her to the center of the cross rail. Walk or even stop her at it if you have to, but do not let her run out. That's a very easy bad habit to start and much harder to fix. When all else fails, think more leg and remember you can never sit too tall on approach to a fence.
Once you get basics down, you can add other jumps beyond the cross rail, such as a low vertical 18' from the cross rail. Your instructor should be able to help you with the proper spacing.
You just have to see your distance...you don't have to like it.