Bobby Fischer famously said „All that matters on the chessboard is good moves“, but in a practical game stamina and psychological factors are also very important. After all, if you are tired or thinking about different things, you won’t be able to come up with good moves. So there are many things other than studying chess which will help you to improve your play.
Physical Fitness
It’s well known that being in good physical shape is important for chess players since you need a lot of energy to play a long OTB game. However, I think that being in good shape – and therefore having more energy – is even more important if you are an adult improver and need the energy if you want to study chess after a long day at work.
Fighting Spirit
When watching grandmaster games, it’s noticeable that they don’t give up easily when they’re in a worse position. One memorable example is Carlsen-Jones, Wijk aan Zee 2018 where Carlsen loses a piece early on but came back to win the game.
In worse positions, you shouldn't give up and play half-hearted moves. This can be very difficult from a psychological standpoint, but one thing you can try is to find something about your position you like (a good knight outpost, the bishop pair or something similar) and try to revolve your play around this favourable imbalance. One strong knight or a space advantage can cause problems for your opponent and it may lead to a mistake by them.
Trying to gain the initiative, even at the cost of material, is also a very effective way to save worse positions. It's always difficult to defend and even more so when your opponent had a clear advantage and they have to switch from a comfortable game to defending against an attack.
You shouldn't go for cheap tricks unless the game is nearly over and you should also avoid forcing variations unless they improve your position siginficantly. Giving your opponent more options means that they have to make decisions which costs time and they might falter along the way.
Concentration
Being able to concentrate at the board is very important and helps you to extract your best chess. I have written a complete article about how I try to improve my concentration which you can find here.
Confidence
Being confident in your chances can also lead to better results. If you don’t believe in your ideas, you will start doubting your play and this will result in worse moves. Second guessing your decisions after they have been made is also harmful for your play. So you should always try to believe in yourself and your play.
This can however be hard to balance since too much confidence will result in underestimating your opponent and their play. You should never think to yourself that you will win a game just because your opponent is lower rated or in time trouble. You always have to make an effort to come up with good moves and take your opponent's resources into account.
Further Resources
The performance coach Benjamin Portheault was recently on the Dojo Talks and How To Chess podcasts and talked about the same topic, so you should definitely listen to these episodes.