To a point I can totally feel this. I had only gotten fit as an adult and felt great about it. I used to work out almost every day. Then I got myself into running, went from not being able to run continuously for twenty minutes to running 45 minutes three days per week. Was in good shape. But in more and more pain.

Pain got so bad that I went to see the doctor. Got told I had a physical issue that is not curable and would only get worse. I got told I wasn't allowed to go running ever again. Or some other activities I enjoyed doing.

Felt defeated. Broke down crying when I got home.

This point on is where our story differs. I put time in finding what still works for me. Found sports and activities I hadn't really done before but was able to do with my condition. Worked with my doctor and a physical therapist to see what helps. Now I still work out almost every day, it's just different sports or exercises but the same amount of time spent.

As to losing fat: that is lost in the kitchen.

You said in your comments you couldn't stick to 1800 calories a day, but the calories by themselves are only part of the equation. I know some recipes that make you three pounds of food/1.5 kgs that contain less than 500 calories. It's so much to eat you can't possibly finish but you feel like you are overfilled all the time so it's easy to stick to. You can talk to a nutritionist to find out what forms of eating work for you.

Now I don't know your pain so I do not have advice on working out. I don't know if there are ways to improve your condition and I don't know if sports are even recommend for you.

But in my experience, keep talking to the doctor and if you can afford it or it's covered by insurance, also work with either a physical therapist, good personal trainer and possibly a nutritionist to get started and find what works for you in the long run.