There is more stress than you can possibly imagine. This is not something people do their entire lives. The heart can't handle it. You will be demoralized beyond belief, and will probably leave your celeb's employ a hollow shell of your former self. But that's really Hollywood in general.
Especially because most Celebrity Personal Assistants have low self-esteem in the first place, the stress can be magnified. You may do a lot of crying into your stuffed animals at night. Basically, if you give it a shot and realize you can't take the emotional ups and downs (mostly downs), then get out. There's no sense in driving yourself to an early grave.