Modifying one of my earlier posts in this thread to make it relevant to knees :)...
going on top can be very painful in long bursts, especially if your knees are weak and your muscles fatigue easily, but how about only doing it for a few minutes at a time? you still get to do it, but then you can switch to a more comfy position!....though on particularly bad knee days I'd avoid it all together.
I find going on top, then leaning down over him so I'm laying on him and him doing the thrusting can make it less painful on the knees too as it shifts the weight from the knees to the upper body.
Doggy can be less painful on the knees if you alter it...instead of all foors, lay down on your front and use a pillow to raise your hips and bum....even more intimate and sexy than doggy with half the pain....
Sucking cock? I lie down on my side on a pillow and get my OH to put his cock in front of my mouth, can suck just as well without the knee or back pain and less muscle work required! I also find, if he sits on a low setee then I can sit right between his legs so I am sitting more than kneeling, less painful than kneeling up whilst still having a sexy angle.
For 69 we lie side by side and I can use his leg as a head rest...this one is a little more awkward for him though.
As for the mental side of it all - it's a tough one - knowing you struggle to do something that you feel like you should be able to do is a horrible feeling. You really have to try and adjust the way you view these things. The fact that you want to make your sex life work despite your problems is something that many perfectly able bodied people don't think about. A happy relationship takes a bit of work and you've got the head start because you've got a reminder that sometimes take a little work :). And with adjustments you don't have to miss out on things - you just have to plan when and how you do things a little more carefully and prioritise the important things.
I find it helps to be open with my partner - he's so understanding. And I've really helped myself by realising sex isn't all about penetration and orgasming. It's about intimacy and connecting as a couple so if one day I can't manage sex because it hurts too much, we can have a gentle blow job, a naked cuddle or he can come in my mouth if I'm really, really sore and can't manage anything else. But I always prioritise some form of intimacy on a daily basis because I think it's important to our relationship :)
Hope some of this helps :)
Adx