Vertical shin with straight leg

During the "vertical shin" stretch against the wall, with the opposite leg straight on the ground, this places a LOT of pressure on my patella or just proximal to my patella, and is so painful that I'm not able to stay in the stretch for more than 10-15 seconds. I am just using a yoga mat against the wall. Do I just need additional padding? Any other tips?

Incidentally, my patella bones are pretty sensitive and this has been an impediment to getting my front splits as well, as stretching in my splits while placing any weight directly on my patella is very uncomfortable. 

Hi Amelia , have you tried do perform 2-3 set of squats in order to properly warm up the patellar tendon?

I use a thicker pad, my patella does not like the pressure either.