I'm sorry your OS caused you so much pain, but it is critical you get your extension back early. It is far more difficult to sort out than flexion and lack of extension will lead to further problems not only in your knee but also your hip and back. The ripple effect can be debilitating.
Things to help extension:
Prone hangs +/– ankle weights with warm packs on hams & calf for 10 – 30 minutes
Heel props (i.e. heel/ankle propped on a cushion/firm roll) +/– ankle weights placed above the knee for 10 – 30 minutes
Extension stretches where the ankle is propped on a cushion and you press down on the thigh and release repeatedly 30 – 60 times, every two hours if you can
Quad sets starting with simple ones then increase hold to 2s, 5s, 7s, 10s as tolerated – 3 sets of 10 3 – 4 times a day
The extension will affect flexion to begin with so it's best to alternate your extension exercises with flexion and end with extension. If you have significant swelling try to adapt what you can so that you can have your leg elevated with the knee above heart level.
Your should also be doing patella mobilizations.
TBH your physio should have been doing these things with you and showing you what you need to do between sessions.
Have you looked up Dirk Kokmeyer's piece of rehab for arthrofibrosis. You can find this by putting his name in the Search box at the top right or going to the Information Hub.