It was a mix of struggles for me, firstly when designing and implementing mechanics the lack of plugins brought up some interesting challenges for me.
I had decided to make my game with 0 plugins, so some features got scrapped or would take too much time to implement so were unfinished or not included. (I actually managed to stick with my 0 plugin goal though!)
My own motivation would go up and down as I tried tackling different aspects of my game. I had some really high days and some really low ones. Some days I didn't work at all because I was so stressed. There was a lot of balancing my own mental health with the progression of the game.
Lastly, the biggest struggle for me was the mapping. Just getting myself to make the base shape of maps was a struggle. I've always had a hard time with maps and level design as I rarely get ideas for them, so I work off a blank canvas most of the time with no direction. I also wanted to make my mapping allow for more exploration then I'm used to which was another challenge. I'm not certain I did a good job with my maps, but at least the game is playable and I got through it. :)