Yesterday I played a tournament game against a lower rated player. I was white. We entered a Sicilian sub-line of which I didn't know much. Soon, my opening advantage evaporated and I started feeling uncomfortable, even worse, although the engine insists that it is at least equal for me. Because I was so pessimistic about my position, I went on to sacrifice the exchange in a complicated line I calculated beforehand. I knew it wouldn't work, since I had seen his response in advance. Nevertheless, I guess I hoped he'd go wrong somewhere. I lost soon thereafter, because I was down material with no compensation. I am convinced that my mistake was of psychological nature. Maybe I wanted to really show that I'm the stronger player by finishing him off with a combination? But yeah, it didn't really work. Maybe my pessimism also contributed to this, I don't know. Is there anyone who has experienced a similar problem and can help me out?
It's a very natural human instinct to go "all-in" when we feel cornered. Sometimes, that reaction actually succeeds because the enemy is taken by surprise.. sometimes all it does is make us lose faster (which might still feel better than continue suffering). There are many situations where it actually makes a lot of sense in chess: For example, if you are already down material, you are more or less forced to go on the offensive. If you don't, your opponent can simply grind you down and win the endgame with that extra piece or pawn.
However.. that is not what happened in your case. Instead, it sounds like you created your problem yourself. And I'd argue that, rather than pessimism, you displayed the wrong kind of optimism. You hoped that your opponent would not see the same moves that you saw. DON'T DO THAT. Ever. If you can see a move, always assume that your opponent can do the same. After all, if you are matched against each other, you will likely not be far away from each other in terms of rating/skill.
It is always good in chess to be pessimistic about your opponent's moves. This is the "MinMax" way of thinking (if you want to know more, google it ;) ). It's the mindset to improve in the game, rather than just (maybe) swindle your way to a few victories you did not really deserve according to our silicon friends.
I've had similar problems, generally with higher rated players though. I assume they see things I can't. But the advice I've seen is to play the board and not make assumptions. If you're going to lose, it might as well be with your best play rather than defending against threats that may or may not be there. If they are and you can't see them, then you'll just learn thereby. Otherwise, you'll do better than you would with an ultra conservative approach. To give a specific example, I played in a simul against Tigran Petrosian many years ago and lost a piece trying to defend against imaginary threats. Subsequently I relaxed, thinking I was lost anyway, and played better, just reacting to what I saw on the board. I even made a combination that won his Q for a R and B, although I still lost in the ending. But I thought I still learned a valuable lesson as a result, i.e. to play with confidence. In other words, I'm confirming your belief that attitude is very important.
You still make around 50% points in somewhat unpleasant position against weaker player by playing normal chess. I guess you don't make so much by naive assault. Otherwise it would be correct play under any circumstances. Prepare for these scenarios as well as for Sicilian or French. Think about them in training, when there is no stress involved. The result will be better when you prepare strategy for such situations with good mindset.
Playing with lower rated Players & Winning against them can be a serious topic. Even I have played with lower rated Players and have been defeated . The lower rated Players have something all have in common . They will mostly in 90% cases will deviate from the main lines of Ultra Sharp variations like Sicilian , French , Caro Kann, Pirc and basically lines with e4 variation .
In d4 majority of the openings are harmless and it is somewhat impossible to make out an advantage in the openings unless you have a 2200 higher ELO .
I have read in Books how GMs tackle the lower rated opponents . Basically they allow the opponent to make the mistake and then they pounce upon it . They surely know that their Chess Skill is higher and eventually one mistake would cost the lower rated opponent dearly and make them loose the Game . In your approach you were first not confident of the opening line and then with fumbling attitude you sacrificed the piece with knowing the best response from your opponent . Basically you were trying to win by luck here thinking that because he stands less on the rating side he will do a mistake .
My advise for you is :
Do not judge anybody by the rating .
Practice with lower rated opponents online . It will help you to grasp their ideas and fortify your skill against them .
If you are playing e4 then you can make a subset of your opening repertoire or narrow it down like for an example when someone play Caro Kann against me I play the Panov Attack . I am very uncomfortable in the other lines of Caro Kann or most probably I have not yet mustered .
Nowadays with the advent of Computers , apps and other digital technology everyone has a great opening understanding . You cannot only just concentrate on Middle game or End game . You need to make a good opening choice else how can you stand well in the Middle game ! Your psychology will be optimistic & strong when you make your pieces stand on better Squares .
"I went on to sacrifice the exchange in a complicated line I calculated beforehand. I knew it wouldn't work, since I had seen his response in advance. Nevertheless, I guess I hoped he'd go wrong somewhere."
This. You did not play the board, you played the opponent's rating. You need to be circumspect and look at the board and play the best move you can. If you see a sacrifice opportunity and you see it does not work, look elsewhere. Don't play "hope" chess.
Don't worry. You'll do it again, and lose. It takes a few iterations before you realize that playing good chess is actual work.