lunging forward with my knee bent and then leaning back and stretching out the leg
Lunges are very bad for a new knee. They put way too much stress on this baby joint and they are keeping you inflamed and swollen. Even if you don't see much on the outside of your knee, the inside still has considerable swelling. It's a very small space in there and any amount of fluid will hinder the ROM.
I’ve been having hydrotherapy
This can also be bad on a new knee. In water, the resistance of it is like using weights on your leg. This is also a no-no for this baby knee of yours.
I know the answer will be stop doing this as my knee obviously doesn’t like it, but how do I improve my ROM if I’m not exercising.
You don't need to exercise it at all. Many of us didn't and still came out great and with much less pain and swelling than those who choose to exercise. I never took PT or did any exercises. What I did do, though, was to take care of my needs, the house, the yard, and my cats. In other words, my ADL, (activities of daily living). That's all I did and I recovered nicely. At first, I wasn't able to do much but as I healed I was able to do more. That is all the exercise my knee needed to rehab itself. Your knee knows how to do that all by itself, too! I'm not the only one who did or are doing only ADL and our recoveries are very good without the added pain and swelling of added physio.
Your recovery sounds like it is coming along very well. I think you need to either stop all the extra exercise you are doing or cut way back. This will give your knee time to heal. Remember, that's what you are doing, healing, not in training! You have plenty of time to achieve the ROM you
want, not your therapist. Give yourself a pat on the back. You've earned it!