fold/call with four to a flush

Just wondering what you guys do when it comes up. Last night I had A-K, board came King,10, blank. I raised furiously with 2 callers on flop and turn. The river completes a straight for me, but it also puts four to a suit I don't have - D'OH. The one caller bets out, and I'm 100% sure I'm beaten so I fold cos surely with two other guys there has to be a flush out there- WRONG, the winner had a King high straight so I would have won.
Well I was so pissed off I couldn't carry on playing. I guess I've learned my lesson about folding on a big pot when you're certain you're beaten ie: DON't DO IT

That said I'll be calling those four flush boards for the rest of my life and losing hundreds of $ ... sigh....