I experience the sciatic pain behind the knee when I try to stretch my hamstrings or sit in the floor.

First of all this tightness is so bad that I can't sit on the floor with my legs straight out. A lot of the spinal stretches and hamstring stretches require you to do this. But I genuinely can't- not even close.

How can fix this....? I feel really lost because I'm starting from such a low point. I'm still in my 20s. I hope there's a chance for me to improve. It's been this way since I was a kid. I've always remembered not being able to sit with my legs stretched out in front of me.

I've been doing the seated nerve flossing but I can't even straighten my leg during this. That burning pain behind the knee is eye watering.