My name is XXXXX XXXXX X will be helping you today. Most times an issue like yours is a defect with the spool valve inside the power steering. This is the valve that controls how the fluid pressure is used to assist steering. The valve will get a slight leak on one of the seals on one side, which then makes it harder to turn that direction as you do not have full pressure. The issue will vary in severity depending on where leak is in comparison to how you are turning. Sadly, the only way to repair a spool valve issue is to replace the steering rack. The resistance will be higher at lower vehicle speeds as there is more resistance at the wheels and less pressure from the pump (lower engine speed usually).
If your problem was both directions I would look at the pump, belt or fluid issues, but since it is only one direction, it does limit the possibilities.
Hi Forgot to say that accompanying the "sticking" there seems to be a clunk sound from the general direction of the power steering when continuing to turn the steering wheel. Would this also be explained by the sticking valve.