Severely difficult to turn

My power steering fluid reservoir has been leaking for a few months now. I have usually put in roughly one quart of power steering fluid a week. Then putting Lucas stop leak for power steering and the problem stop. A couple of days ago while driving The steering got extremely difficult to turn steering wheel. The car is still drivable however it takes two hands to make a turn and it's very slow and extremely stiff. I need to know what to do to fix this issue. I do not know whether it is the power steering pump the rack and pinion or if it is a belt of some sort. Any suggestions on what I should do to get it fixed.