Yeah I have this too.
I'm almost never genuinely classically ill though. With vomiting and stuff. But a lot of times I feel a low omnipresent level of stress and fatigue, also sleep-related. I guess that's depression. I bite through it most of the time, but some days when things get really bad I really just want to call in sick, but can't, for fear of being judged as lazy or some sort of guilt for letting others do my work. Also imo these things aren't really taken seriously as a valid reason to call in sick in many workplaces (You didn't sleep last night? Suck it up dude! Most people have no idea what it's like for the body and mind to be chronically sleep deprived), so I would have to fake some sort of "accepted" illness on the phone. I just can't do that anymore.
I guess a lot of it (the mustering of courage) comes down to self-confidence, and also for introverts it's just hard to voice your feelings or show a weakness. If you're ill then that's your call, it's your health and you shouldn't compromise it or feel shy of calling in sick. But I know how hard that can be.
