[Small Fry]

Middle guy limps and then you just min raise. Of course he's calling this. If he is a good player your check on the flop tells him exactly what you have - two overcards, and he's flopped a set. He knows you'll fold to a bet so he's hoping for you to bluff or hit your card. His wish is granted on the river. The pot is pretty big so it's not going to be too east to get away from.

I don't think you really have anything to be ashamed about though. He played the hand well. You don't mention how long you took to call him on the river but don't be afraid to take some time to really think about the hand, the board, the player, your image, his image, etc. before calling.

Next time think about going all in. It puts a lot of pressure on a player with a small pair and takes away their implied odds.

Also, Depending on the player the sigh is a big tell. Acting weak when they're strong. I play against a guy who frequently does the whole I'm weak act whenever he's got a big hand. Makes it so easy to A) not bet into him and B) fold when he finally (and reluctantly- still pretending he's got nothing) bets.