Why do I lose games in the endgame if I won the middlegame?

This is a really common problem. When there are fewer pieces on the board, the position looks simple, and your brain relaxes right when you need maximum concentration. Plus, endgames require completely different skills - what matters there is the king, pawns, tempos, the square rule. It's easy to lose focus if you've been playing actively and aggressively the entire middlegame.

Start by just learning the basic endgames. King and pawn versus king, simple rook endgames, pawn endings - this isn't boring theory, it's a toolkit that saves you from making mistakes out of nowhere. When you know that king plus pawn should be winning in a position, you won't start making thoughtless moves out of doubt. Solve endgame puzzles - there are tons of apps and problem collections out there, takes 15-20 minutes a day.

Second is the psychological side. If you won the middlegame and feel like you're starting to rush or relax, take a pause before the endgame. Literally acknowledge it: okay, everything changes now, this needs a different strategy. Switch to calculating each move slowly, don't rely on intuition. People often lose a win simply because they start playing on autopilot. Even in a winning position you need to stay serious.

What exactly happens in the endgame - are you just moving too slowly, or do you completely lose the logic of the position? Relaxation after winning the middlegame is part of the problem, but not all of it. Often it's the opposite: psychologically the tension rises because a mistake in the endgame can literally lose the game in two or three moves, and because of that fear you start playing timidly, without a plan. The endgame differs from the middlegame not so much in the complexity of calculation as in the clarity of the criteria - the king becomes a strong piece there, pawns matter more than a rook, and any imprecise move is immediately obvious.

Try not to speed up in endgames, but instead slow down and each time work out a plan for yourself two or three moves ahead: where am I taking my king, which pawn am I defending or attacking, how does this lead to a win. And definitely study the basic positions - king and pawn versus king, king and rook versus king. It'll take a week or two, but then in practical endgames you'll feel much more confident because your brain will stop panicking when it sees an empty board.

The problem often isn't relaxation, but the fact that endgame principles - an active king, advancing pawns, every tempo matters - are a completely different language than the middlegame, where you're used to attacking and combining. When there are few pieces, your intuition stops working because there's no intuition there - only calculation and knowledge of typical positions.

Try to figure out exactly where things fall apart: are you missing your opponent's counterplay (not seeing their activity in response), or do you not know how to correctly push a pawn, or are you losing time in time trouble? A specific reason will give you a specific solution - learn tactical defense, study typical endgames, or just play more calmly.

Pawn endgames and king plus minor piece versus king positions - that's where you should start, because there's no noise there and every move can be calculated to the end. I noticed that once you start seeing the logic in simple positions (where the king needs to go, how the pawn moves), other endgames become much clearer - they work by the same principles, just with more pieces added. It's not just about relaxing; it's also that you haven't yet developed an automatic feel for these positions, so every move takes mental effort that builds up. Literally an hour a day on endgame positions from a textbook - and in a week or two it'll become clear what's happening on the board.