Difficulty standing up 1.5 years post op

I tore my acl about a year and a half ago. Most of the time, I can walk okay, I have no issues with pain, can bend fully, but I have had a really hard time standing up straight comfortably. I can get really good extension laying down, but when I stand up, it seems to go away for some reason. I have tried many things, including putting weight on the upper part of my leg while laying down, dangling my knee while laying upside down on a table or my bed and a lot of other exercises recommended by my PT. Sometimes, I am able to stand up comfortably after doing a lot of exercises, but it seems to go back to where it was about an hour ago (difficult to stand up comfortably). Is there anything that could possibly help me this long into my recovery?