I'm sorry that you are struggling. I hope the tests yield some results that will lead to an effective course of treatment soon. Keep at the doctors to keep investigating!
In the meantime, it might help to go back to basics. Track your food. Experiment with your exercise level to figure out what you can do NOW most consistently, and re-calculate your calorie requirements to account for less activity.
Consistency is the important thing. It's hard to re-adjust your goals when you are young and you used to be able to do more. But, it's also hard to be discouraged by having goals you can't possibly reach in your current situation. Once your doctors find out how to treat you, you can increase your exercise goals. But in the meantime there is no use feeling discouraged because you can only do what you can do.
I'd also probably focus on strength training to help with the muscle loss, even if cardio seems too challenging.
Take care, I hope you get some answers soon!
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"People often say that motivation doesn't last. Well, neither does bathing--that's why we recommend it daily." - Zig Ziglar