Why do I play so bad?

Probably the answer is quite easy: I don‘t practice.

On the other hand I was a little bit better some month ago.

But it could be that this was just an illusion.

And it may be I jumped to some wrong conclusions.

Regards

Its a real problem to me

Its like going to the gym and lifting. Keep playing properly and you will see results. There is no point of asking why you're still skinny after a few weeks of training.

@Makropoulos , I am no expert by any means, but I have thought a lot about this aspect of chess. Players who play but don't get any better. There are a lot of them out there.

After looking at your profile, I noticed something. You only play correspondence. While after a little thought it may seem that this would be best for those who want to improve, because you have so much time to think about the position, but I have found that this is not the case. Correspondence simply takes so much time, and is so slow paced, that the active learning process developed by playing rapid and classical, is never able to get under way. So, here is what I suggest that you try.
Play the other time controls. If you don't like bullet and blitz, don't worry about them, but to be honest with you, after playing lots of them for a couple weeks and coming back to rapid, I went up close to 100 points.
In any case, make sure that you play rapid, and or classical, but that is not all. After each game, win or loss, you should go to the analysis board and analyze your own game, by yourself, for a while. Then after that, you can turn stockfish on and go through the moves again, taking careful note of where the evaluation changes a lot, and seeing why accordingly. Finally, take a look at the learn from your mistakes feature. Sometimes it helps, sometimes not. I am convinced that if you do this for enough time, over time, you will improve a lot. At the level you are at, you could probably get to 1200-1300 in rapid and classical without a huge amount of rigorous practice, etc.
Hope this helps. If you have questions feel free to ask me or anyone else!

@Makropoulos It will also help you to do the lichess training puzzles. To get to them, go to the learn tab at the top of the page, and from the drop down list, select puzzles. The system will give you puzzles and ask you to find the best move. Generally something that will win you material(i.e a rook for a bishop, a queen for a rook, two minor pieces for a rook, etc).

You seem to play correspondence only. That is OK. So can your question be rephrased as why you plat badly at correspondence? How long do you analyse over a move? A former International Correspondence Chess Fereation Worlf Champion saod: "In correspondence chess he who goes to bed later wins."
Let us look at an example

Why do not you recapture his central pawn at d4? You let him have a pawn as a thorn in your position, hindering the natural move Nc3. You stay a pawn down with no compensation, which in correspondence means a sure loss. Afterwards you trade pieces even to a lost pawn ending. If you are down material, then trade pawns, not pieces for drawing chances. Over the board that may help, but in correspondence it is hopeless. So you lost the game as early as move 4.

@Makropoulos Learn chess like you are circuit training:
Do at least 100 chess puzzles and then and only then pick chess players that have a similar rating.

Consider your puzzle rating to be your skill level. It's your milestone for progress.
Create games that are ±200 rating points close to your puzzle rating. Analyze those games.

As you play more strategically created games and solve more tactical chess puzzles you will see your rating progress.
Widen your experience, and look at the views that match your opening. Gain a balanced experience of the game. Play the same opening pawn structure for at least 25 times, then move onto another pawn opening structure. Hone what you already know and add to it.

Try to strenthen the basics. That basic knowledge you might be tempted to skip for being so "easy", has to be known by heart. Nowadays there's plenty of basic videos on youtube, but you have to apply that knowledge on the board. You can watch a video of someone giving mate with two bishops and think that it's easy, but can you do it? Can you mate with Queen or Rook + King vs Lone King, everytime, without stalemate, with time pressure? Try doing this, make sure you know all the most basic stuff by heart, then see if you notice a difference after a couple of weeks. Be patient.

I don't know how to improve in chess, I'm only interested in playing for fun. However I do know that chess is like most things where just turning up and playing isn't the _fastest_ way to improve, you will improve, it's not the fastest way.

If you're interested in improving I would suggest you do some research on a more efficient way to improve. Basically, learn how to learn chess. No doubt there are many different ideas and some might be more suited to you than others.

Also, even the world number 1 has a coach, or many coaches. You can still improve without one but it might be a bit slower.

Do you analyze your games? If you simply play games but never analyze the results (especially your losses) then you are only going to receive a fraction of the benefit of having played a game that is available to you. This site provides a wealth of useful information after completing games, both computer analysis and multigame analytics. There is a wealth of information there that can be reviewed to point you towards where you can improve.

Others have mentioned solving tactical puzzles. Absolutely yes. This site does a great job of presenting puzzles that will allow you to have both success and challenge. The higher your puzzle score, the more challenging the puzzles you get to solve. I have also heard from many good players that one of their favorite pre tournament training tools is solving tactical puzzles. This can potentially transform your ability to see opportunities in your games that you would usually overlook.

Playing games is obviously useful as well. Reading about chess but never applying it would be fruitless. You are playing games - are you actually trying your best during them? If not, then you can't expect to improve at all - it's just pushing wood.

Not everyone wants to truly try to improve. Some just expect that there will be some slow, steady improvement over time. This may be true to an extent. The fact that you are here asking why suggests that you'd like to see more consistent improvement. Could you try to do the things above for a time and then see if that is making a difference? If not, then I believe you are destined to simply hover around a set rating.

Thank again, you could just have suffered a few tough losses, which happens to everyone. At the end of the day, if playing brings you joy, then you are blessed. Nothing wrong with that, at all.