Hello Dolphin, It sounds like you're feeling let down again and I'm sorry you're going through this. Just remember, even when it feels like things won't improve, they still can. You made it through your last "blip", as you described it, and there's always hope, even if it doesn't feel like it right now.
BUT my ROM is EXACTLY the same as 3 months 4 days ago.
You noted that
"despite EVERYTHING I've done, nothing has made any difference. If I'd increased to 50o I'd have the patience and the determination to keep going."
Measuring range of motion is not an exact science. Those tools have limitations. There can be inconsistency in placement each time, swelling can affect measurements and human error is also possible. Pain may limit how far you're willing to move the joint and fear in doing so can skew measurements. Different people may measure slightly differently. ROM changes from day to day.
I've come to the conclusion that my new knee needs to be done again. Unfortunately it's a failure
Slow progress does not mean your knee needs to be revised. It can take months and progress isn't always straightforward. Pain, swelling and muscle tightness can restrict motion and that doesn't have to mean your TKR is a failure. You can still have a successful outcome. Only a qualified person can determine if your knee has failed. Your lack of ROM likely indicates you need to adjust any therapy and continue to address pain and swelling.
Please don't assume you have an infection without lab work. Telling your OS's support team that you think you have an infection won't move your appointment up. They have protocol to follow and unless there are clear clinical signs, they can't / won't bump you ahead of others. There is always urgent care if you feel your situation is that dire and you cannot wait.
I was going to take my dogs out, first time, and blow the consequences. I didn't but am planning to do so today. I have to for my mental health.
I am not understanding why you'd do this if you feel it will have a negative effect. I hear that you're hurting, but threatening to hurt yourself isn't a healthy or safe way to get support. I get that waiting for appointments feels frustrating, and it isn't ideal, but turning down appointments, or arguing about the timing, isn't going to help things move faster for you. Sometimes the best option is to take what you're offered and stay on their list in case something opens up sooner. It's not about settling, but keeping the process moving. Again, there is 111 for emergencies.
I think the idea of getting outdoors for a short walk in the fresh air, taking in the sounds of nature and feeling the warmth of the sun, if it's out, will likely help your outlook. Taking the dogs right now is probably not a good idea. Possibly your neighbor / walking partner will accompany you. Hopefully with some icing, elevation and fresh air, your day will feel brighter.

Best Wishes!