Ooo boy, do I know that feeling. And I'm a chronic complainer (though I try to limit it to my journal and therapist). I realized how bad it got when my (now ex) friend told me she had the precursor to lupus, and she went on and on for weeks about how she would have to change her life to avoid it/delay onset. I just wanted to tell her how what she was talking about was nothing compared to how my sister lived (and she was only moderately disabled- though she did spend about 20% of her life in the hospital). I felt horrible for thinking all that, though. I didn't have lupus, and had no clue what she was going through. I did lose sympathy, though, when she didn't take care of herself. I bought her a hat and always offered her my sunblock for an outdoor PE class; she always refused them (and the hat looked cute on her), among other things.
Just after my sister died, I was even worse! A neighbor had broken up with her boyfriend and spent a lot of time crying and beeing needy. I thought "pff, she broke up with a guy after a year, I lost my sister of seventeen". But I remembered the last time I had broken up, and how hurt I was. Just because I was going through something worse, didn't mean it didn't hurt her.
But yeah, I definitely know the feeling. Sometimes I feel bad about it. Other times I'm greatful that I don't take these things for granted. And if I ever did lose them, I might be more prepared because I'm always thinking about how people who don't have x ability get along. It's like seeing miracles that others can't. I know to find joy in abilities to communicate and walk, etc. My therapist told me never to feel guilty for a thought. If you didn't act on it and hurt someone's feelings, and you recognized that it wasn't right so that you would not act on similar thoughts in the future, don't beat yourself up. Plenty of people would not be so kinds as to keep their mouths shut.