Contrary to what you might think, the currency of poker decision-making is not money, at least not as we know it. In the real world, money is a stable currency that reliably represents the worth of what we buy and sell. If I sell my car for $5000, I'll get $5000 for the car. I make a transaction for X dollars and so I gain or lose exactly X dollars. In poker, things don't work like this, not in the short-term. You might make a raise that on average earns you $15, but end up losing $100 because of bad luck on that one occasion. You lost $100 in money, but you gained $15 in EV.
So what exactly is EV?
'EV' stands for 'expected value' and is the real currency of poker decision-making. As we cannot control the monetary result of our poker transactions in the individual instances in which we make them, we need a stable currency that does not fluctuate with short-term variance. EV is simply the amount of money we gain or lose on average in the long-term due to our poker actions. Some plays are described as '+EV' meaning that they make us money in the long run, while others we call '-EV' because they'll costs us money over a large sample. EV is what we're trying to maximise each and every time we have a choice to make in a hand.
So basically luck is entirely irrelevant. It does not matter, whether you gain or lose money over one hand, or 10,000 hands for that matter, as long as you gain EV over that sample. The upshot is that in 100,000 hands of playing in a +EV way, there's a very good chance you'll have gained money. This is what beating poker is - a long term grind, churning out EV knowing that sooner or later it will become real money.
Let's imagine that you played a 500 hand microstakes poker session one afternoon where the sum of every choice that you made earned you +$8 in EV. In the short-term, there exists a ton of possible monetary outcomes for that session. In fact, this figure grossly underrepresents just how many different possibilities there are. Some days you'll win more than $8; other days you'll win less than $8. On more rare occasions, where the natural variance in poker has been against you over this small sample, you'll even lose money. Your EV is still +$8, the actual results are just down to variance.
Always Look for the +EV Move!
What we should always try to do as poker players is look for the best move (the move with the highest expectation) at any given time. If you always make the best possible move, you'll always make money in the long term.
There will of course be times when you make the best move and still lose; that is inevitable, since poker is a game that is influenced by variance. Bad plays are going to get rewarded, but that doesn't all of a sudden make their moves good.
Put the short term completely out of your mind and focus only on making the best decision at any given time. Good results are destined to follow.