jayareemcee I take it Steam is removing all the DLC you don't own, then reinstalling it when your girlfriend signs in? There's no fix, per se, but I can suggest a workaround. Rename the "The Sims 4" folder that contains the game's program files, here by default:
C:\Program Files (x86)\Steam\steamapps\common\The Sims 4
and download another copy of the game with the other account. When you want to switch back, rename the new copy and restore the old one's name. The idea here is that you'd have two copies of the game installed, one each with your DLC and your girlfriend's, and you'd switch which folder is called "The Sims 4" depending on who wants to play. This might take some getting used to but is a lot faster than reinstalling all that content.
When renaming the folder, make sure Steam and the EA App are closed and their respective services are NOT running in the Task Manager's background processes list.