..... I'm sorry you're feeling low. I know the slow progress is frustrating. Even if it makes sense that you would take much longer to heal with all your knee has been through, it doesn't help when you want to be back at living life again. I hope you can find joy in the things that are
better now than months ago. Sometimes it helps to write down any small steps of progress (like walking around the garden center among beautiful plants) so you can read them later when you might not be having such a great day. There are going to be plenty of ups and downs, but hopefully the "ups" will be in the majority soon.
It's okay to grieve the loss of your old knee and whatever mobility you had with it. That's pretty normal when a person is having a lot of post-op challenges. But the reality is that, had you kept your knee, it would not have remained as you remember it. Arthritis doesn't work like that....it's degenerative (meaning it continues to get worse). By this time you most likely would have bone damage from the surfaces rubbing together constantly. That can be extremely painful and eventually result in a person needing a wheelchair. I believe as you continue to make progress, these feelings of regret will lessen.
Once your knee has had a chance to completely settle (that could be up to a year from now) and you know the infection is under control, you could seek out additional opinions on your flexibility. It is possible that you have developed scar tissue (adhesions) that are restricting your joint bend. There is a surgical process for removing adhesions, but it takes a REALLY specialized orthopedic surgeon to do it properly so they don't return. Just keep that thought in the back of your mind as you continue to slowly increase your daily activity and regain strength you've lost in that leg and your whole body. The good news is that you are in London and you should be able to find such as specialist without having to drive long distances when you're at a point that you want to check into that option.
Right now the goal is to heal and settle those soft tissues. Sending you ((((HUGS)))) and strength!