Before yo do anything, make sure that the problem is a result of the opener itself and not due to a hangup in the travel of the door when it gets near the bottom. Disconnect the opener and operate the garage door by hand to make sure the door operates smoothly and feels balanced. A properly balanced garage door should remain in the half opened/half closed position all by itself without trying to fall or rise by itself. If THIS is in order, rehook up the garage door openers 'arm' to the garage door and proceed with what I say next:
Don't you have a newer style auto garage door opener with a safety sensor that will prevent crushing? They are supposed to retract automaticaly before some little kid's head gets crushed, for example.
Even if your 'sensitivity' setting on the garage door opener, is set to minimum sensitivity, it still should STILL not be crushing the door.
But, before giving up hope on this opener, first look for the control for sensitivity on the opener (it will be on one of the sides or bottom where you can see it), and try turning the knob in the direction of "increase sensitivity", and see what happens. It may also be listed as " up force" on one knob and "down force" on another knob, and you turn these in the appropriate directions as they will either list escalating numbers or show a widening arrow band. You are interested in the "down force" one.)
Garage door openers also have an 'up travel' and a 'down travel' adjustment knobs. You should also try to turn the down travel knob so that you get it to where the door won't even close all the way. Then adjust the knob so that it hits just enough to squish down the door bottom weatherseal a little bit. You actually should try to make this adjustment before you even do the sensitivity adjustment.