I'm making a lemon mousse filling for tomorrow's cake with whipping cream. Is there something I can add to it to keep it from separating?

Perhaps you need a different recipe.

Here's a lemon mousse recipe with gelatin:

http://www.ladycakes.com/Recipes/Fillings/Lemon%20Mousse%20Filling.htm

HTH

it could be the heat! it happened to me once! i went through 4 batches of cream before i realized what it was. trick is to use a metal bowl and chill the bowl in the freezer and the beaters before whipping the cream. the aim is to keep the cream cold. if you dont have a metal bowl, put the bowl your whipping the cream in into an ice bath. i also like to stick the cream in the freezer for 5 mins just before i whip it.. can get reallllly hot here! but doing all this ive never had another instance of it!

Thank you everyone, for your responses.

Scarlets-Cakes... step-by-step lemon mousse with pictures:

http://www.pastrypal.com/2009/07/21/lemon-mousse-in-60-seconds/

In the comments, there's also someone who has a different method to make it, that also sounds smart.