Chronic illness is assumed to be predictable. If you, the patient, follow the advice of the medical professionals, take your medicines, eat well, get your exercise, avoid triggers, then you will effectively "manage" your condition.
I used to think of myself as a Runner. I'd pore over race schedules and the calendar and plan out running routes for weeks, even months, in anticipation of a big race. Now I think of myself as someone who gets to run, sometimes. This week, I can't run. I may be able to run next week, probably the week after that ... but who knows?
Here's the earliest definition of "patient" in the Oxford English Dictionary: "Enduring pain, affliction, inconvenience, etc., calmly, without discontent or complaint; characterized by or showing such endurance." The use of the word to refer to a person with an illness appears about the same time: the very definition of "patient" is intertwined with the notion that one will suffer, but should do so in silence.
I come down with a cold or I come into contact with an allergen, and who knows? My body might shrug it off, or it might go into dragon-slayer mode, my lungs retracting and twisting into inutility. One day, I ride a bike or climb a mountain; the next day, climbing a flight of stairs may be beyond me. I have not gotten accustomed to the suddenness of the shifts, though the extent of the changes surprises me less than it once did.
During episodes, there's fear. How long will it take to recover? Will this episode leave me more impaired than before? Between episodes, there's fear. What will trigger the next? When might it come? Will I become unavailable to family, lose time at work, miss out on vacation activities? Managing the stress becomes a constant part of "managing" the illness.