FROM D McB:
Well, it's pretty obvious that someone made a mistake in giving you your current responsibilities, to begin with. But don't be trying to foist the consequences off on some other member of the staff. You're going to have to take responsibility for your actions, gallop on over to your superiors, and own up to your mistake, offering to do anything to set it right. You're quite likely correct: most sensible Office Managers will see that you deserve to become that replacement. When that happens, don't stamp your hooves or kick up a fuss, just present your flanks for the harness. It may not be an appealing outcome, but it is the honorable thing to do.

FROM G:
By the time this is published you probably won't be in a position to read it. After all, if you cannot make yourself choose a PA to be a replacement it's because you don't want anyone else to do it. Admit it; being a pony girl is an attractive release from the stress of your job.