Tunnel vision
This game illustrates nicely what a lot of beginners have a problem with; being so focused on one plan and one section of the board and not seeing a simple retort or a counter attack. It also illustrates to me what a lot of people succumb to when a game goes like this: despair and hopelessness.
I think white resigned too soon. I was only slightly ahead in material by then and with mistakes all around, he might have fought his way back to at least a draw (if not a win). But losing a winning position can be deflating, and sometimes the fiercest opponent can be yourself.
I'm proud of winning this game (not necessarily the way I played it) because even when I felt like hitting the resign button, I didn't give up. I played it out.
Still, lessons to be learned here. If nothing else, this game was instructive.