sims 3 crashes at loading screen

@mandee_mack  Yes, you can copy the folder for a particular save into the new user data folder.  Each save has its own folder in Documents\Electronic Arts\The Sims 3\Saves, and they're labeled by the name you use for that save.  There's also a second copy with a .backup extension, which contains the files from the second-to-last time you saved.  If you ever need to restore a backup, you can delete the .backup extension and change the name (so you don't have two folders with the same name in the same location), and it will be available to load from the Main Menu.

It sounds like you might be downloading bad custom content from the Exchange.  This is common—some bad cc sticks to uploads, and many creators don't know, or didn't know at the time, that they had any in their own folders.  If you're interested in downloading a lot of content, you may want to check it out in a clean folder first.  You can install it, check it out in-game, and make sure the launcher is still working.  Definitely scroll through the launcher's Installed Content tab looking for any items you didn't mean to download: you might see something that doesn't belong, or the launcher might crash.  Either way, you'll know not to install that particular content into your main user data folder.

It's pretty easy, once you get the hang of it, to move your main Sims 3 folder out of Documents\Electronic Arts (or rename it temporarily), open the launcher to spawn a clean folder, download a couple items you'd like to use, install them and test, then trash the new folder and put the old one back when you're satisfied.  It's also a lot easier than starting from scratch with your custom content every time.