If someone will call 2 streets with an underpair then value betting is IN, IN, IN!!!
Stuff happens, sometimes the deck just keeps kicking you in the ass, but if you consistantly value bet the best hand then over a large enough sample you'll make good money.
As for dealing with it in the moment, you gotta find what works for you. For me, I usually just take a deep breath and remind myself that I WANT to play against people who will call to the river with crap hands, because over time they're my main donators.
Probably the wrong idea is to chase losses until the early hours of the morning when you're emotionally affected by it though.
