My older game disappears whenever I load a new game

@Zoeibell  Each save is independent from all the others, so any sims you create in one save won't show up in your other existing saves or in new ones.

If you want to add new sims to an existing save, you need to load that save first, then go to Manage Worlds and into CAS from there.  You can also save sims to your library and place them in other saves, again from Manage Worlds.