This is how the Prestige mechanic works. When you start playing the tutorial directs you to upgrade the sawmill to show you prestige mechanics.  It resets your progress at each new level (sawmill) but gives you a higher income multiplier and more opportunities.
You keep diamonds, reputation, biz points, chests, managers cards, and your Workshop. Based on your previous experience and the new managers, you might develop your new sawmill differently.

Please note that you can't come back!