I was better, then I was getting checkmated and didn't know what to do. What can I learn from this?

10 O-O-O? is risky and the root cause of the later checkmate. Better 10 Ne2.
12 Nh4 is the right idea, but better is to capture hxg6 first so as to open the h-file
20 Nhg7?? goes downhill. The knight is useless there. You had to play defense 20 Kb1, which you nearly always need to play after O-O-O. You played it on move 26, but by then it was too late.

Also, I think once you realize that their attack is coming you might need to break in the center. You could have prepared or played e4 a few times I think.

"I was better"
When?

You moved your bishop twice in the opening. You changed that bishop for a knight. You "ruined" his pawn structure, but that structure most of the times is an asset rather than a liability. That exchange didnt really get you much.

I know is an opening, but you really need to know what are you doing there instead of just regurgitating. The bishop in general, is more valuable than the knight. That opening is not for you. It really is more advanced, as it puts you at a disadvantage in the long term, unless you actually get something for the bad exchange.

Then you allowed the other bishop to be taken by his knight. So thats 2 bad exchanges for your bishops.

You had the extra pawn, but you had no attack. You never opened the position, you were busy putting moving the knights around doing nothing.

The knights in the 5th rank are generally good, but it doesnt matter where they are if they are useless.
Chess is a square control game, yes, but its not about controlling the most squares in either side of the board, its about controlling the important squares. The knights in the fifth rank and above are only good if they are actually impeding movement or restricting key squares to the opposing side's pieces. Your knights werent doing anything, not even a slight discomfort. If anything, they were blocking any kind of penetration or break from your side. They were playing against you. So you wasted quite some tempos on positioning them where they were useless. You couldnt even sacrifice any knights to open the position as he did.

You never had anything there. If you are gonna get something from this game 2 things.
1st is to never trade your bishops for knights willingly.
Only when you are forced to do so.

2dn. Put your pieces in useful squares. Its fine if you have to maneuver a couple of moves. But maneuvering pieces to useless squares for several turns will only hurt you.

Now, the question of how to identify useful squares is the answer that most of us seek and we cant find. But generally speaking, you do have to create disconfort and treats. If the king is chilling without worries of getting checkmated or to lose material, then its a useless square you control right there.

@Alientcp said in #4:

"I was better"
When? ...
From moves 12 to 18 ?

https://lichess.org/bf6lpgnR

Don't provoke him, unless you want to hear "Condescending Gibberish: part II". I don't.

@MusicGarlic said in #6:

Don't provoke him, unless you want to hear "Condescending Gibberish: part II". I don't.
Its not condescending, Im not criticizing him as a player. He just didnt had anything going up.

@kindaspongey said in #5:

From moves 12 to 18 ? lichess.org/bf6lpgnR
Doesnt matter what the engine says. Its what you see on the board.

You say you had the advantage from 12-18, but in fact, you did offensive maneuvers from 12-20. 8 moves. Then you did other 2 after.
He only made 1 defensive move, Bf8.

Lets see his offense to compare.

20.- Ba6. You are forced to a defensive move.
21.- Reb8. You made a defensive move.
22.- Then he can go a4, which is a slow move, but he can afford it because he isnt in danger at all. Then you feel like you have time and play f4.
23.- Bc4. You continue to try open the position.
24.- He sacs, you recapture with the king
25.- b3, you make a defensive move
26.- a3, you make a defensive move
27.- a2, you make a defensive move
28.- He captures your queen, you recapture.
29.- he mates. You cant move anymore, but if you could, you would do a defensive move.

In the same 10 moves, you made 7 defensive moves. He made 1. That is how pressure looks like.

Honestly, you had nothing. Had you played f3-d4, Rhe1, he would have been forced to make defensive moves, you would have opened some space for a probable queen infiltration, Ne7 could have been an option once the lines opened, maybe even e5 if he didnt captured.

It is unclear if you had won the game that way too, as his pieces would also come alive, but at least you would have a chance of starting an offense. But as the game went, his king was chilling. you had no access to diagonals (thats why keeping at least 1 bishop is useful), and even if you did, you were blocking them yourself with the knights. You gave his king a protective layer with your pieces. And to be fair, not all games end in checkmate, sometimes you have to play for a passed pawn, or just mere control of a file or diagonal. Even a tactic, a pin, a fork, a discovered check, anything. There was absolutely nothing there.

Im not trying to be rude, its just a fact, there was no clear path to victory there. Not even a hint of an advantage other than the pawn.

You are supposed to give your opponent issues, you had 10 moves, he barely reacted. So, you might had advantage according to the engine, which has enough elo to figure out that there was an advantage, we dont have that elo, so if we cant spot that we have an advantage on the board, then we dont have an advantage.

So, what I think you should take from this game.
Bishops are generally more useful than the knights. That opening is very advanced as it willingly gives up a bishop early, but you dont have (neither do i) the level to get something in exchange for that. If you cant get anything for it, dont make things easier for him, you wasted 2 tempos and gave a bishop for that knight.

While it is true that there are sometimes were we are forced to lose a bishop for a knight, try to avoid it when possible, but dont lose the pair for both knights that easily, you will have nothing to challenge their bishops, as it was shown here.

Aim for the weak pawns, open the position to get an attack going.

If your opponent is not reacting to your attack, could be the case that there is nothing there.

It was a bad game, just notice that you didnt had anything there, your opponent's king was safe at all times, go next, try to make an actual attack next time.

And lastly, dont pay attention to what the engine says. It will say that you have x advantage which may miss lead you, as the advantage is only real if you manage to follow it up with very specific moves that you most likely wont find. The engine evaluate the position based on the useful squares that you control, we tend to evaluate by material. So the engine eval does not necessarily correlate with your eval, there are different criteria used. So, the eval is invalid if you dont play for the same strategy as the engine.

For me, despite the pawn advantage, the game was equal until move 20~. Thats why I object to the statement that you had an advantage. You didnt showed anything for it.

It was just a bad game, dont fixate too much on it. You never got an advantage. Just go next.

Just keep these two things in mind:-

1. Offense is the best defense.
2. Flank Attacks are countered by opening up the center (this is why engine is pleading you to play e4)

I also feel you need to calculate a bit more, and develop a feel for initiative. For example, to me, it seems obvious that you want an open file for your rook therefore you play Hg6, and you played Nhg7 because I think you simply didn't see Ba6.

Also, the Bf6 move in the opening is surely bad whatever the engine says, as it allows black more space, engines can play without space, but for humans, more space=better position.

@Alientcp (starting with a quote of MusicGarlic) said in #4:

"I was better"
When? ...
@kindaspongey said in #5:
From moves 12 to 18 ?

https://lichess.org/bf6lpgnR

@Alientcp said (apparently to MusicGarlic) in #7:
... Doesnt matter what the engine says. Its what you see on the board.
... Honestly, you had nothing. ...
What would Alientcp have done (as Black) after 1 Nf3 f5 2 d4 Nf6 3 Bg5 g6 4 Bxf6 exf6 5 e3 d5 6 Nc3 a6 7 Qd2 Bg7 8 Bd3 O-O 9 h4 Re8 10 O-O-O Nc6 11 h5 Nb4 12 hxg6 ?

White was indeed winning from move 11 up to move 20.
However, after 20 Nhg7?? instead of 20 Kb1 black is winning.
It is common for players who stand better to believe they continue to stand better, some kind of mental inertia.