You can start a new save game file, add all your builds and sims that you have created to that save file, then save it with a title such as “master copy”. The next time that you want to start a new save file, load that “ master copy” save file, but then choose to “save as” with a new name, then you will make a new copy that you can play with which is separate from your master copy. You can then keep going back and using the master copy each time, but using the “save as” the first time after using the master copy.