I've been a manager/head coach a couple of times, men's softball (player/coach). I think the biggest mistake I made was overestimating the dedication level of my players
The first time I coached I got frustrated because I didn't anticipate that the "weekend warriors" on the team would be content to just show up on Friday or Saturday night and play the game and socialize, and wouldn't want to do any practicing. I also had a number of players who didn't get the concept of responsibility when it came to letting me know if they couldn't make a game. They'd just not show up. The second time I did a better job of setting expectations - both my own and the minimum I expected from the team - and it went better. I still found it to be more frustrating than it was worth though -lol- It was amazing how many grown men needed me to act like their father, which was not what I wanted to do.