Resigning in a won position

I've just played a game today as White and I was in a tough position where my opponent was threatening to win a piece and had a nasty pin against my knight on d4. I was worried that I would lose a piece after he played f6 to threaten my bishop on e5. I had just realised that he had left his e6 pawn undefended so I took it with check. I didn't realise at this point that I was in a winning position and I resigned because I thought I was going to lose a piece. But I did some computer analysis and it came up with the Nh4 idea, which I didn't see.

How could I get good enough to see these kind of moves and how can I avoid these types of positions?

And when I say how can I avoid these types of positions, I mean the initial position before he left his pawn undefended.

In the position after 18. f6... your knight on d4 is in the danger zone. I don't like these positions with "hanging pieces", they require accurate play to avoid mistakes that lose pieces. After 16. Rfd8... I can sense the future pin as a potential problem, so if you leave the knight there in a pin you will constantly have to check for tactics where your opponent can utilize this pin. In the position after 16. Rfd8 it's hard to imagine what else you can do, but in similar positions i would recommend getting your queen out of the pin as it is unpleasant. To avoid getting similar "unpleasant" or "difficult" positions i would recommend playing more calm and closed games where there isn't that much tension and action going on in the centre. A solid pawn chain position I think. You should keep protecting pieces and sense when your pieces are in danger. Exchanging pieces will reduce tension and make it easier to play good chess. Chess is simple, but also very complex!

And you should't resign when you think you might lose a piece, if there still are pieces left. Try to complicate the position (even if you don't like it) and make life hell for your opponent. The more pieces on the board the more chances. In games with lower time controls I think this is a good idea. Don't go defensive! If you are a piece down does it really matter if you lose 1 or 2 pawns more? The answer is no! Try to do something, create threats, be annoying, try to create a trap and even bluff. Maybe the most important thing is to keep queens on the board, they increase drawing chances massively! Try to exchange as many pawns as possible when you are a piece down in endgames or middlegames. 1 less pawn = 1 less queen. Many players does't know how to checkmate with knight and bishop! Be resourceful and creative. Be alerted a blunder can happen at any time.

I hope this can help you...

ofcource he may have resigned due to some unavoidable work.

@BobBanana2 part of the journey to becoming a great player involves becoming increasingly comfortable with tension-filled positions! though of course, hanging pieces is not tension, that's an invitation for unfriendly tactics.

You may have resigned because you had some other work that time probably.

Nobody ever won a game by resigning.
MIKHAIL TAL

"When you are on the attack, never resign"
Me

"How could I get good enough to see these kind of moves ...?"

Keep on practicing.