If your problem is a lack of ability to control the extension in the range of motion from fully straight to about 30 deg of flex with the one knee, I'd experiment with a temporary "assist" by doing something like the following, whatever works, until you can improve your control...
1. use the opposite leg as a limited "assist" to help the initial lowering until you get past the uncomfortable range, then continue without the aid
2. wrap a velcro band around your ankle and attach a long strap to it and loop it over a heavy chair back next to you, and use the strap as a control to help lower the leg throug the challenging part of your range of motion, limiting the assist to the minimum needed to help, letting your leg do as much as it can, decreasing the "help" as you improve your control.
I can't advocate to try this or not, but if it were me, I'd be thinking of something like this to see if it might help me to get a grip on controlling the last bit of range of motion. Ask your therapist and see if something like this might work...
