Hard to say without flying the plane myself, but I have several overpowered trainers and I have no trouble taking off straight. I suspect you may have inadvertently applied some left aileron or rudder. I've been guilty of this when I was first learning. If you have a computer radio, you can increase your "null" zone using the exponential settings and it might help keep inadvertent stick movement from having any affect. If you are using the "high" rate, try using "low" rate and see if it helps.