It sounds like either your rear wheels, your center of lift, or both are not close enough to the center of mass. If your rear wheels are too far back, they'll push into the surface instead of allowing the aircraft to raise its nose. If the center of lift is too far back, aerodynamic forces will prevent your aircraft from acquiring an adequate AoA for takeoff.
Ensuring your front wheel extends further down than the rear ones also ensures that your aircraft passively gathers lift as it accelerates.
You can also decrease the friction control and brake force on the front wheel as well if you still have problems with the aircraft spinning out, and if that's not enough you can do the opposite to the rear wheels.
There can never be too much thrust
Probably because you designed it asymmetrically or messed up a setting on one of the ailerons. Have you also tried doing Alt+X? That'll reset your trimming.
Bej Kerman's post in Unintentional Airplane Drifting! How do I stop it?! was marked as the answer