I think the hardest part for a 2 year old is that they have no concept of how long is "over night" so .. maybe if you have time .. try explaining in terms she'll understand like . .. I'll be  home by the time your done with watching "whatever her favorite t.v. cartoon to watch at a set time". I found out the easiest way to ease their fears is to be consistent ... let's say you went grocery shopping and left her with a baby sitter  and you promised "I'll be home before .... " and you leave .. when you come home when you said ... they learn to trust your word ... so I have no doubts this transition will be alot harder on you .. then her, you've always been there for her and she's knows that .