One day at a time. Since you cannot cure that illness, I recommend fixing everything in your life that you have control on and that might help you feel better. Sleep well, eat a good diet, skip fast-food and alcohol when possible, etc. This won't cure it (don't listen to people that say it will) but it will make your overall life better. This will make it so that when the chronic illness hits you, you're at a more st…
Your baseline will be at a different place than someone who is healthy. You must learn it and learn to have realistic goals. You should not try to push yourself past this, it will burn you. Especially if that illness is new and you are used to how you were previously. You won't be the same, and that's alright. Don't aim for "healthy person + 10%". Instead, aim for "your reality + 10%".
What is important is to learn to aim for a little above that baseline. Push yourself in realistic ways. Hit the gym even if you don't want to. See a therapist even if it feels like a chore. Do all the thing you won't want to do. They are hard and it's okay. Especially if you were down for a long time, you probably developed bad habits. It's okay to be lazy at the start but you need to find your cruise speed.
Push yourself as hard as your illness allows. If that means only being able to go to the gym once a month and taking short walks on most weeks, then so be it. But stay active, both of mind and body.