I don't know if you read the whole article that one guy linked, but here's a good part you should read.
Your knees hurt because they are taking a beating. Try repading the ski with a lot of underpadding to absord the shock. 25mm? is like 1inch thick and could help a lot. Also, try that exercise and see if you are distributing the force right.
If you ride quads, race quads, and ride them right, you are not at the extended position you describe. You have to get way down on the side of the quad and have your knees bent 45 degrees or more and shoved into the front fender to properly corner. Maybe you're riding utility quads and that's why your knees are extended. If that's the case, then I can understand where your comming from when you say you want a couch. Utility bikes are the couches of MX.
I understand you hurt yourself, and I can't say how bad it was or how you should have recovered. i can however comment on my own injury, which I had at 19, where I obliterated my right foot. I was on a dirt bike and shattered 3 of the metatarsals of my foot. I was immediatly put into surgery, which most of the surgery was removing fragment and some of it was splinting, it was bad. It took 2 years to conjure up the testies to get back on that bike. i can assure you, that every amount of pain I have felt since I broke that foot, has been due to the lack of rehab. I have not recovered fully in muscle tone and I don't take a normal stride which causes probelms. All of those problems are correctable. I (and in your instance, probably that is) need to spend more time rehabing.
I don't know about you, but I took my injury like a champ. I look at the things that hurt as a problem with me. No injury is going to hold me back. Maybe your injury is really than bad, maybe you need to man up, or maybe MX and stand ups just are not for you. That's your call, but if your riding sport MX, you can ride a stand up, you may not like it, but say you don't like it, and not that you can't. Can't is for **************. Not liking something doesn't make you a puss.