i was exactly in your shoes. i sold a company as a single founder and after i sold it, it felt good and left a big void in my life that i tried to fill with other new ideas but was never quite the same. i think part of this reason is that, when you have a successful business, you know every little aspect of it to the point that other businesses doesn't hit it to your heart. but once your next business takes off, it'll be all the same. it's just a matter of time.