I'm sorry to hear this, but my I'll awnser in 2 ways:
1) Having the best hands pre-flop (ore even just befor the river) doesn't mean you'll win if the opponent has any chance left (even a 1% or less) there "must" be times that they will win!! (Just rules of statistics (a bit of Murphy's law), and when you're the one that has go get the blow it sucks)
2) PLEASE, PLEASE, PLEASE,,, learn to handle youre bankroll with "BANKROLLMANAGEMENT"; if you've got $20 in youre bankroll you should play $2 games!!!!!!!! (if you lose that $2 and therefore left with $18 you should play $1.5 games,...) This way you can play longer with youre monney and avoid the losses of some bad-playing-lucky-guys(/girls). And the only way to improve youre skills is to play (a lot).
To close this post, look on the other side, lets pretend you're on the downside (ex. you AK opponent AA) and still win, this gives you a rush. If you want to beleve in balance of the universe winning here and losing there is all part of life (sertenly in poker).
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