Also, instead of putting in the celebrations disc, i have to put in the Pets disc in order for it to load.
Normal behavior -- the manual mentions that if you have Pets or a newer EP, then you use that EP disk rather than Celebrations to run the game.
The 'loading until it had a little way to go' and then crashing is usually the sign of an incompatible global hack, or a bad clothing or hair mesh. Not sure why it would cause problems now when it didn't before, but you should make sure all your hacks are up to date, and if that doesn't help, then you'll have to take all your downloads out and do a binary search to find the bad hack or mesh.