I'm 63 and have had knee surgery also so can relate. Arthritis is pretty common at our age unfortunately.
I agree with Kitt. You should get a proper assessment regarding the flexibility around your knee as you may have some imbalances. Knees can be adversely affected by muscle imbalances above and below the knee.
Perhaps in the meantime, simple exercise like a wall sit or leg raises (lying on back, one knee bent with foot on floor and the other leg straight....lift the straight leg up and down a few times, then lie on side and raising the outer leg up and down....then changing legs and raising the leg that's on the floor up and down to work the inner thigh, then lying on your stomach and raising the leg up and down while keeping hips on floor). This will get you started and will be easy on your knee as there is no movement during the exercise. Please ask your Doctor first to ensure it is safe for you at this time.
Kitt mentioned osteoporosis. I didn't see that in your post but if you do have that, just be sure to support your spine well in all the exercises you do.
I hope this helps and I'm sorry you are getting frustrated. Just listen well to your body. If your leg is very weak, take things slowly. Slow will win the race every time!!
Good luck Sandie!