You can try a 10x5 or 11x4 prop, it might help.

But I agree with Minn, check your wing and stab incidence. Also check the thrustline of the engine compared to the wing and stab as ekrcflyer suggested. I don't know for sure in your case, but most trainers should have tad of down thrust.

Double check your CG on the wing. If your wing is somehow too far forward, and the CG mark is on the fuse, you can wind up tail heavy.

Check your ailerons for too much reflex or droop. This could easily cause the problem.