Patience and mental fortitude to deal with variance. Some bad runs last so long you might think you're just a terrible player... it isn't always the case!
This is a very important take on it.
Poker is based a lot on confidence. There is a reason why players on winning streaks tend to win more. When all you correct cbets gets called or reraised, and your correct bluffs gets called a tad too many times in a row, and you lose a few buy ins it often feels as if none of your plays are correct.
This is something that breaks a lot of newer players, since they may have a boosted ego due to a run of good luck, and then once the variance turns, they feel as if they have no real shot at winning in poker.
Once you've been through it a few times, you learn to appreciate the good runs for what it is, and you don't feel too bad when your kings or queens are shoved into better hands.
As you get more experience in this game, you end up developing a different skillset that makes you a lot more profitable than other players in a long run.
Although a simplified version of reality, most poker players after some point plays pretty much in the same way. The only difference is that more experienced players will realise when to get to showdown cheaper with good hands that can be beat, and when not to tilt when faced with bad beats, since what comes around goes around.
