Who decides that I am mentally incapable of making personal care decisions?

It depends on the situation. In most cases, your attorney would decide this unless you name someone else in your Power of Attorney to "confirm" that you are mentally incapable.

If you do name someone else, your attorney cannot start making personal care decisions for you until that other person confirms that you are incapable of making decisions. If your attorney thinks you might be mentally incapable, they must arrange for that person to assess you and confirm your incapacity.