I began chess by playing mostly 3/0 games. I did this for a few years. Yes, long games are very difficult, you have to learn how to actually calculate... and analyze. Analysis is comparing one candidate move to another, then picking the best candidate move based on the lines you've calculated for each of them.
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Anyway, other than playing longer games more regularly (I joined a chess club) I solved tactic puzzles with a focus on accuracy. No moving the pieces, solve completely from the original position. When you're completely finished calculating a line, write it down. Then if there are any alternate tries for the opponent, write down how you would beat those too. Always without moving the pieces.
Try to see the end position clearly. Write down specifically how much you're ahead in the end position like "I win a knight for a pawn" not "I win a piece."
The goal is to find the whole solution... not just the winning idea, not just the moves for you side. You're trying to find the best defense for the opponent too. You want one of the lines you wrote down be the exact and total solution.
What this is doing is forcing you to calculate slowly, visualize clearly, and render an evaluation... generally none of these are done in a speed game.
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If the problem is too difficult to solve, don't guess and sacrifice a piece for nothing. You can write down the line, but be sure to mark that you think it's losing for you. After that, calculate the best line you can find as if it were a real game, and make that your solution.
I have been playing chess for 2 years (Well, actually you can count that as 1 year as I have not played much for the past 11 months or so). The bulk of my games were bullet games and blitz games. My style for quick games is to "sacrifice as much as I need (or at least as much I THINK i need) to be aggressive, keeping calculation to the minimum. Now, I realise that I have a lot of difficulty moving to the "slow thinking" mode and bring my bullet style to standard games, which of course, has costed me many games. In addition to being unable to go to Standard chess mode, I have also lost my ability to play chess as well as before. Is there a better method to resolve this issue than to simply play plenty of standard chess games and tactics? Please help and thank you in advance