What I'd do differently
The biggest mistake I made was selling without an M&A adviser. Not because of the price, but because of information. I like being open, and in a negotiation openness about your valuation, your deadlines and your reasons to sell is exactly what the other side can use. An adviser would have helped me give away less.
As for why founders sell at all, Tomáš Čupr put it best: either you're bored, or you know more than the buyer does.