I'm glad it's not just me thinking that! I trained as a yoga teacher many moons ago, but haven't practiced in years. I am now on Day 5 of the yoga programme and at best would consider it intermediate. In some places it's advanced. The only way you could reach that level, especially before you became disheartened, is to consistently repeat the earlier easier classes. For beginners and remedial pupils I would always advise additional hands on teaching to prevent bad habits and injury. Yoga, when done well, is exceptionally restorative and becomes not just an exercise programme but a peaceful way of life. Yoga was part of a programme that returned me to health after a very long illness.
Good luck with it!
Great tip from Mandy
Perhaps some basic yoga (easy class, short duration) with modifications and using props such as blocks and blankets to support your knee. Whilst there are easy classes on this site there are not a lot of modifications so I would advise finding a class that suits your comfort, learning the name of the pose and then looking online for any modifications. Better still, supplement your classes on here by attending a remedial yoga class and after a while, modifying your poses will come instinctively to you and your body. Hope that helps :)