Can't is a word I heard a lot while learning to ride.
I have heard you can't:
Overcome my timidness
Barrel race on a Thoroughbred
Well guess what? I have now not only ridden bareback at the walk, trot and canter but also ridden tackless over a jump and around a barrel. I have galloped full out on an ex-race trained horse. I have jumped up to 3'3". I have gone to many shows on many different horses and brought home some nice ribbons. I have overcame a lot of my timidness and have branched out to riding not only my horse but a Clydesdale, a Gypsy Vanner that is still in training, a stubborn Paint and many others. I may still be timid on a new horse but I am more willing to try now than I was before. I also barrel raced by Thoroughbred at a show and we did just fine. Being told I can't used to hurt me in the past but now I just look at people and say oh really, watch me.
For me all it took was that one special horse to show me I can.