Game won't start after pressing play on the launcher

WHY DID THAT WORK???? I mean, I'm happy about it and all, but HUH??? IT WAS A CLEAN INSTALL! I had already backed up and deleted everything! So, here is my question now, what do I do with all of my saves and custom stuff? Is there a way to test it without having to restart every time a new item is added? Let's just say, I have a bit of custom stuff. As I said in the post, I've been playing it since 2009, and I LOVE a few creators...  😄