My list of essential resources to study.

Mathematics of NL Hold'em video series on www.deucescracked.com by WiltOnTilt
The Eightfold Path to Poker Enlightenment series on www.deucescracked.com by Tommy Angelo
Book "Easy Game" By Andrew Seidman from www.balugawhale.com
Book "The Mental Game of Poker" by Jared Tendler from mentalgameofpoker.com
Bart Hanson's Podcast at www.seatopenpoker.net

Results Don't Matter

This is a very simple concept every good poker player knows but it’s very hard to grasp for average human being. We all too used to judge performance by results and we are all too result oriented in our life. But the fact is result of a single hand of poker does not matter. What matter is who made a mistake playing this hand? If you make (bigger) mistake then your opponent you lose money in the long run if he made mistake – you profit from it in the long run. 
To demonstrate this concept lets look at a very simple game. We make a deal to flip a coin. You put $1 on heads and I put $1.10 on tails. Assuming coin is not rigged and your probability of winning is 50% it does not matter which side the coin lands on every specific toss, this game is profitable for you in the long run. 
Same with poker. Poker is not a card game. Poker is the game of making deals. And if you make more profitable deals then your opponents than you make money in the long run. Cards in poker only used to bring variance in the game so players who make bad deals could win sometimes and that keeps them playing. 
Not being result oriented also helps your mental game. If you concentrate on making most profitable plays and don’t care about results of each single hand you won’t be affected by bad beats. Because you know if you got bad beat somebody had made very bad (and very profitable for you) deal with you. And you are happy about it.

Variance is my friend.

I love variance. Variance is the main reason that it’s much easier to make money at poker then at no variance game like chess for example. Variance is making sure that loosing players win sometimes and keep coming back to play again. Variance fueling delusional people’s beliefs that they are good at poker because they win sometimes and only bad luck doesn't allow them to win constantly. Seeing variance unknowledgeable people believe that poker is a game of luck, which is great because it gives them reason to not improve. Nobody needs skills in the game of luck.

When I embraced variance I realized that having more variance in a game is great for me. So now I’m trying to generate it when game is lacking it. For example in bad 2/5 tight passive game full of short stacks hand might play out this way:

MP (300) limps 5h5d
I fold Qd9c in HJ
CO (250) limps 7c8c
BTN limps, blinds complete. Pot $25. 
Flop Kd5c6h. Blinds check, MP checks to trap, CO checks, BTN checks.
Turn 9s. Blinds check, MP bets $15. CO calls to trap. All fold. Pot $55.
River 7d. MP checks scared of straight. CO bets $25, trying to get call from top pair. MP calls. Pot $105. Casino takes $10 rake (10%). Stack sizes now: MP $255, CO $345. Is this good for me? No, because now it’s $10 less for me to win at the table.

What if I try to introduce a bit more variance in this game?
MP (300) limps 5h5d
I raise Qd9c in HJ to $25
CO (250) calls 7c8c
BTN folds, blinds fold. MP calls. Pot $82. 
Flop Kd5c6h. MP checks to trap, I bet $35, CO calls, MP raises to 70, I fold, CO calls. Pot $232.
Turn 9s. MP bets 75, CO goes AI for 155, MP calls.
River 7d. Pot 542. CO wins with straight. Casino takes $15 rake (2.8%). MP reloads for 300. Stack sizes now: MP $300, CO $527. And I’m loving this. Yes, I just lost $60. But what do I get in return? 
1. There are $285 more at the table for me to win
2. MP is super tilted that he lost with flopped set. It’s very likely he’s now going to spew some more trying to get even.
3. CO is now full stack and he’s not used to play deep. He’s happy, orders another beer and will tend to chase his draws for any money, reinforced with positive result.
This outcome is great for me. Also I don’t really lose $60 here in the long run. Next time flop comes Kd2c6h and I take it down with cbet. So this is win-win situation.

Do I want to reduce variance in my results? Obviously, yes! But what’s not so obvious is how? I believe the only way to reduce variance is to increase your win rate by increasing your skills advantage over your opponents. Thinking of extreme example: if you have opponent who plays with his hole cards face up. It’s going to be pretty hard to lose in this game. Your win rate will be very big and variance is very low. (You still lose occasionally when you go all-in preflop with AA vs KK for example). So, to reduce variance I thrive to study poker off the table as much as possible. And it pays off in the long run.