1. for now, do couch stretch with a chair/stool/actual couch where you can hang the foot over the edge of the support. it will thus not be fully extended but bent at the ankle. this should relieve the tension somewhat until you adapt.

2. start with the torso angled lower towards the knee that is up. keep the torso locked in good posture and then come up. come up only to the angle at which you can still maintain good posture. do not go to the point where you start overextending the back. from your description, i’m guessing that you probably feel tension way before vertical, if you are maintaining proper posture. only come up to this position until things loosen up. i’m guessing that glute contraction is easier if you are more bent over versus vertical. that is another good sign for the short term; come up to the angle at which you can maintain glute contraction during this stretch. of course challenge yourself with more vertical angle as your ability to hold glute contraction increases. 

3. put a finger or hand on the rear glute and squeeze it. the hand/finger will give you tactile feedback that you are contracting and maintaining that contraction for the full duration of the stretch. work on keeping that contraction the whole time. at first, it will probably waver a lot. if your anterior structures are tight, they will neurologically be inhibiting the glutes to contract, along with typical 21st century glute activation problems. but use your brain to squeeze them as tight as possible. eventually you will be able to hold the contraction as the anterior structures loosen up.

4. try to press the hips forward instead of pushing back against the foot. this will put more stretch on the hip side of the quads and may relieve pressure at the knee.

5. voodoo band the quads/lower leg around the knee. they may be tight in general so loosen them up.

good luck!