Thinking about the short term
Of course it is down hearting when you made a great play but you still manage to lose to that one outer on the river but you always have to think about the long term. Your luck will even out and the next 20 times your opponents has a one outer he will probably not hit it and you will be left with the rewards for your great play.
I hear many players say they are going to fold Kings pre flop because they have been beat with them 3 times in a row. You cannot think like this, KK is the second best starting hand and more times then not, if you get your money in you will come out on top.
Attaching yourself to hands
This can also waste a huge amount of chips to new players. When you look down and see AK, sure it is a great hand. If you do not hit with that AK on the flop and you are facing a big bet, you must always fold. Players get too attached to the hand because they just cannot see how such a good hand like AK cannot hit. They still feel they are in front and waste numerous chips on a hand which is going nowhere.
Stay focused and play smart. Sure these sorts of hands are great pre-flop but if you completely miss with the community cards, you will almost always be behind.