Game completely resetting between saves

This sounds normal to me. New Game means starting over, without any houses, community lots, or households that you added or otherwise changed. This makes the starting conditions the same as any other new game. It also means you will start at 0800 (8 am) on Sunday. The only thing that should change is the season, if you have Seasons and chose a different season.

You don't have to start over to add new Sims. You can Load Game, choose an existing save, then click on an unused house or vacant lot. The large button at the bottom right will let you Create New Household To Move In. There's another button at the top right for Manage Households and you can use that to move existing households. You can even do this right after you finish building, by pressing F1 or going to Manage Worlds instead of going back to the Main Menu.

To summarize, use Load Game if you want to keep the changes you made. Use New Game only when you want everything to go back to the way it started. Hope that helps.