Yoga is becoming an exercise in futility

I signed up for an eight week Yoga class that meets at 7:30 am on Saturday was the 2nd class. I am really trying to have an open mind about this. The first class went really well I felt. I had some trouble with some of the poses, but was able to modify. Today was a different story. I got so frustrated in one part. There is a chunk of yoga where you are on your hands and knees. I have arthritis in both of my knees, with my right being worse than my left.  Because of this I have a really hard time kneeling. I can kneel on both for a short period of time, but I am unable to kneel just on my right knee without PAIN. Today the instructor had us do the hand and knee balance (you can see the move here) and this is where I just started to get frustrated. I did try since we have the mat and we are on a padded floor. I slowly raised my right arm and left leg, and then quickly put it back down. The right knee was having none of it. I was able to slightly do the move on the other side but the left knee was not very happy. Another pose that 'should' be easy is child's pose and I never feel comfortable or that I am getting a good stretch. I think I may have to research and see what other modifications I can do if I am going to finish out this class.

Any suggestions or ideas?? I would love to hear from you!