I can't offer much advice. I'm in much the same predicament myself.

My memory lets me down big time and I find I'm unable to make decisions or think things through fast enough in work.
If I don't tell my employer, I am responsible for my actions.
If I do tell them, they are responsible for my actions, errors and ommissions.
I'm a registered nurse; honestly, who wants a fish-brain looking after them? plus I'm in charge of up to 90 residents and can't take a break if needed. things just snowball out of hand if you don't keep up with the work.

If I tell them, I know for a fact my bookings will dry up (agency work). they can't take the risk with the health of others. Or I would need a fitness certificate for each shift.
Either way, i'm stuffed.

Don't rush into telling all. Maybe they;ll think it's a transient situation. people never guess the worst.