Yeah sometimes its hard to play a game right without the pain. I have had that happen to me if that happens try to go and get some water.
Chess is mentally painful. Am I the only one?

Stop playing the computer for a while. Computers play 'perfect' chess, and have a deep opening repertoire. Trying to keep up with that is torturous. It makes you afraid to make a mistake.
Also, try playing simpler games, with a simple strategy. Don't try to comprehend the entire game, this is also torturous.
Remember to have fun. It's a game.

I hate lose also, number 1024. Its very painful, when jou lose from a lowrating player: Lowe rating than me, or, stupid lose less 20 or 30 moves. But when a lose from a strong player and is a good game, than not so painful. I hate lose, when its all the time and many lose gamen again, again and again. I understand this very good, chessplayers!

I also experienced what the OP went through, to some extent anyway. It common in chess because chess is such a prestigious game. Coaches, studying, hours of effort, disheartening behavior, the whole nine yards. It's a grand show of intelligence and wit.
Somewhere along the line it stops feeling like a game.
The solution is simple! Remember to have fun, win or lose. So what if you make a mistake, and so what if somebody else is better than you at chess? Play your hardest and have fun... this way you'll naturally improve.

I used to play very seriously (international level), and I started to hate the game. Like really hate it. So I took a break until last year. That break lasted 26 years. It was a good decision.

Wow! that's quite a break. sounds like a story lurking in there?
Nothing too exciting. Played as a kid, never EVER practiced as was too lazy so under-achieved. Got fed up with the pressure, all my weekends being taken up with chess, and by the age of 13 I was far more interested in football, music girls etc. So I just stopped playing.
Then a couple of years ago I was invited to an informal blitz tournament by some Russian friends. As you can imagine, the standard of casual play among (drunk) Russians is far ahead of what you'd find in most other countries in the world. Plus I'd pretty much forgotten how to play in any meaningful sense. Won 1 out of 10, felt pretty embarrassed. So, I decided to brush up on my tactics and strategy, signed up for the site, and got back into playing in a way that I'd never have imagined possible. It's nice to play purely for pleasure, without worrying about what other people will say if I lose. I still beat myself up for making crucial blunders though!
If losses affect you that bad, maybe you should study chess so you could win more (temporarily). There are hundreds of books for your level.
No matter what my level is, I'll only learn by playing others at my level; the losing (which isn't it anyway) would continue in that case at the same rate. It's also not about "losing" per se. It's the playing of the game. Chess is odd: the playing is brutal. And if I was winning, there would be endless stress about F'ing it up with no reward in the end (see my prior comments). Johnny-Vang (possibly with humor) said it perfectly with "chess is a total head wrecker". At least it is for me.
Therefore I don't think chess is for me. After 50 years of being drawn to it, and experiencing the same angst regardless of being drawn to it, I'm clearly unfit for it.
I'll go remove my account now if possible, but in any case, thanks all.
That is true but if you were 20 you could only get better and have some hope but at your age it's a lot harder to improve or to keep the illusion or hope of improving...i feel your pain...