It's normal for the heli to want to push left due to the side thrust provided by the tail rotor as a side effect of counteracting the main rotor torque. When you're in a stationary hover, you'll probably notice that the heli is leaning slightly down to the right, which is perfectly normal.

The backward drift could be a CG (center of gravity) problem. If you balance the blade grips on your fingers with the nose of the heli facing toward you or away from you, are the skids parallel to the ground?