Help, how do you train against simplest blunders?

The best way to train against simple blunders, is looking for the best moves often ending with a advantageous outcome that benefits you. Sometimes, the simplest blunders can lead to a checkmate for your opponent, so it's always good to look out for good moves. Though there hard to avoid, taking puzzles on Lichess.org can help you improve your skill!

I am not a sports psychologist but I think I know what a sports psychologist would tell you.

1. You are a very good chess player. Your rating puts you in a high percentile of chess ability.
   Positive affirmation: "I am a good chess player."

2. Analyse your best games and note all the good moves you make.
   Positive affirmation: "I make a lot of good moves and I CAN play mistake free games."

Don't think that positive affirmations are corny. They actually do work. They can lift your confidence and performance. A lot of high achievers are very self-critical but this can be a double-edged sword. If used objectively to correct mistakes and push performance higher it is good. But if used to over-criticise yourself and tear yourself down it can be self-destructive.

3. Note your mistakes but don't beat yourself up about them. Note them and move on.
   Positive affirmation: "I will make good moves. I will make correct moves."
   Even repeat this as a mantra before the game launches.

4. Vow to find a method to reduce blunders but don't expect instant success.
   Positive affirmation: "I will find a method to prevent blunders. I will work on it."

You will need to find a method or methods which are recommended or even invent methods yourself to find what works for you.

5. Don't get into a rut. Don't keep beating your head against a wall.
   Positive affirmations: "I will play chess and train in chess in a way which is enjoyable and fruitful for me."

"I will sleep properly, eat properly and physically exercise properly."
(To keep a brain in top condition requires all these things.)

"I will keep my life in balance. I will pay due attention to family, relationships, work, duties and (if you are religious) worship.

Positive thinking can work at real, step-by-step, incremental levels. It's not magic. It will not create instant large improvements. But over time if utilised with creative solutions to your dilemmas it can work.

Rather than "beating yourself up" over blunders find a method to work on reducing blunders and work on that. The mind is an amazing thing. If it knows that blunders will see it "sentenced" to more anti-blunder training (which is less fun than playing and kind of feels like a punishment) it may well "clean up its act" and blunder less.

FOOTNOTE:

Examine your own psychology and see if you can figure out the reasons why you blunder. Mistakes have reasons too. In my case, at a much lower level of playing ability than you, I can see one reason is my over-eagerness to simplify to a won or what I think is a winnable ending. Another reason for me is one-track thinking and only seeing the possible positives of a move without noticing the drawbacks, like it leaves a piece hanging or creates a positional hole.

@Ikonoclast said in #42:

Positive thinking can work at real, step-by-step, incremental levels. It's not magic.

Yes agreed. I used to also add a visualization element where you see yourself performing well in different situations.
I would do this before a game and often at the board.
Identify a potential Problem/ Fear: Think of past situations where you handled this issue well: Then visualize yourself dealing with it well.
For example:
You acknowledge a fear or insecurity such as "I will forget my opening preparation".
You think well this happened to me in a only few games before and I handled it well by just slowing down and working hard to find good moves. You visualize yourself handling this situations well.

You may fear making a mistake and say to yourself that this does not happen often and I can handle this like I did in the past.
You just visualize yourself handling the situation well.

Most of what we fear (95%+) never actually happen. Fears are often irrational.

Chapter 2 General reasons for blundering (p51-97)

in The Chess Manual of Avoidable Mistakes by Romain Edward
Ch01 Objectivity throughout a chess game (p9-50)
Ch03 Concrete moves and concessions (p98-141)
Ch04 A few key tips to improve your results (p142-171)

I just came across this book but I have not read it. It looks interesting.

That book recommended by @bobflanagan1 (in #40) has this idea:
You should not only look for how to win, but also for "How could I lose?"
I've tried this in some games and indeed, it seems it helped me to avoid some blunders.

Apparently aimchess has a blunder prevention trainer. It gives you 2 moves and you have to pick the one that's not a blunder.

I blunder less ...
When I get some sleep,
When I'm happy,
When I'm not distracted.