I went thru this seven years ago, leaving my startup which I'd been with for about 15. I still considered it a startup after all that time. The opportunity had long since been missed and all that remained was a long slow grind down to nothing. Which is exactly what has happened as it continued after my departure.
For me it was as close as I could imagine giving up a child would be like, but in hindsight it was the best choice and I grew hugely as a result.