Standing on one leg requires (relatively) strong abductors, the same muscles that are used in the side-lying leg lifts. When you say lean over though, do you mean to the side? or forward? If forward, your hip flexors might be limiting you as they also stabilize in a one-leg stand which would correlate with your difficulty on supine leg-lifts (which I also struggle with, but it's good enough that I don't have any real functional issues associated with it).
If the supine leg lifts are difficult, try doing them in an easier way. Support yourself if necessary and stand on your good leg, then bring your operated knee upward, leaving your calf dangling vertically. Don't go past 90 degrees, but I'd say it still does the job even if you only get to 45-50 degrees. Hold for 5 seconds and then slowly lower it back down, repeat.
Similarly for stability on the operated leg: stand by something for support, but use it as little as possible, and work on reducing your reliance on the support. Always keep your standing leg flexed slightly at the knee and focus on keeping your torso stable and upright.
I agree with blinky on giving it some more time, there's huge variance in healing time, and prior dysfunction due to the arthritis can wreak havoc on our muscles. Try the things I suggested above as ways to do functional strengthening in a way that you can moderate the difficulty and let us know how it goes!
JD