Physical set backs

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I feel such a failure. This is the first time EVER in my whole life I have felt defeated by my condition.

To simplify what I have i have a muscle problem (called neurofibromatosis) nothing too serious for what it could be (which I am grateful for) but it means I get tired easily and ache faster then others when exorcising. It also can mean a few trips to the hospital as I can have benign tumors growing on my nerves (99% of the time they aren't serious, just painful) Now looking at me you would never guess I has this issue. It's one of those things you cant see - which I suppose I should be happy about. However, I see all of these people at the gym, really going for it, running miles, lifting weights that are heavier then me! I feel like I'll NEVER get to that stage, that its a setback that will just stop me from ever losing s decent amount and toning. Maybe I'm just going through a jealous 'its not fair' thing, but for the first time ever, I have allowed myself to feel incapable.

Has anyone else had to overcome setbacks like this? Disabilities perhaps or just starting from the bottom? Or even some advice for me would be much appreciated!