I type on a keyboard with only my two middle fingers. This was likely more efficient for my tiny hands at five years old than methods urged by the typing CD-ROMs we had at that time. However as I grew more proportional to a standard keyboard, this early typing style has persisted. I have no idea where to even begin changing this now as it’s been deeply ingrained by decades of habit. Anybody have experience with changing a muscle memory based habit like this?
Mavis Beacon was how I learned to type and it’s available for free. Not sure how this version compares to the old one but there were tons of different games to teach you all the different parts of the keyboard (like symbols or the number line).
As for breaking the muscle memory, maybe you can put tape on your middle fingers to try to make it feel difficult to type with just those fingers.