as long as you do not have a trial, previous versions are revealed in the apps tab by clicking the arrow abutting the open/install button
Sorry that you're facing problems using Photoshop CC 2018. Could you please let us know what exactly is the issue you're facing?
We have released a new update for Photoshop CC 2018 version 19.1 and have fixed a lot of reported issues. Please refer this list of fixed issues Fixed issues in Adobe Photoshop CC
Let us know if updating to the CC 2018 latest version helps. See Keeping Photoshop Up-To-Date for update instructions.