It could be reconditioning because it's a long recovery, or it could be you may have developed central sensitization (central = central nervous system = your brain). It happens sometimes to people who have a surgery or an injury, or people with chronic pain conditions. Basically, when you're in pain for a long time, your brain starts to become overprotective to try to prevent you from having more pain.

Normally, pain is a warning sign that something is wrong. But in this situation, your brain starts sending our more frequent pain signals. It's sort of like an overactive car alarm--you feel pain signals even with activities your body should be able to tolerate. The treatment is slow, graded exposure. Or I did something called Graded Motor Imagery.

I haven't had knee replacement, but I'm a caregiver for my mom and she did have knee replacement. And I have chronic pain conditions, including central sensitivity I described above. My mom also had a really long recovery. She had to start really, really slow, only exercising to a pain level that increased no more than 2-3 points on the pain scale, and then she would have to rest for 1-2 days. The PT schedule they gave her in the rehab facility and outpatient was way too aggressive for her. And that's generally the approach I use with central sensitization too; I have to start low and slow.

Have you tried aqua therapy or pool therapy, as mentioned? That can be really great for this because it unweights your body so it can make your body feel "safer," which helps with central sensitization. And if that's not it, it's still really good for unloading your joints and it can give you enough leeway to continue to build muscle, which could be more challenging on land because of gravity.

The last thing I can think of is if you're super-frustrated, could you consider a consult with a physiatrist (physical medicine & rehab) specialist? They're internists and have training in physical therapy and look at the body and muscles more holistically. In my experience, ortho doctors and orthopedic surgeons aren't so helpful once the surgery's already done. I have hip dysplasia and ortho was helpful for diagnosis--not so much for treatment and what exactly I could do to make things better.