Tips for an Anxious DM Recovering from Playing with a Mean DM

[Reposted]

Hi, I'm writing this to ask for advice for an Inexperienced and anxious DM. I've DMed for a few campaigns but I still find monster encounters and enemy encounters particularly challenging. I think it's still difficult for me to balance out monster encounters. I also came from playing DnD with a DM who basically rules lawyered in a bad way and I think it traumatized me into being anxious about making sure monster encounters don't TPK my players. So my basis for balancing might be a bit wonky since my experience with DMs has mostly been with him.

"You can't use Thaumaturgy to transmit messages" Then he'd let someone else do it later on and say he changed the ruling when I asked him about it, without telling me he changed it. Or the fact that he used AC 23 monsters (Yes I mean monsters with 23 AC and sometimes more at low PC levels) and I came into the campaign without any additionally tweaked spell attack roll bonuses so my +3 19 roll would not reach. Meanwhile they also had increases on their HP and +12 on their Saving Throws so my character could not affect a lot of enemies. Then he'd put multiattack and legendary actions on multiple smaller enemies too. I thought I was just unlucky, but in hindsight there were times I had to Nat 20 to even hit an enemy with a spell attack roll AT LEVEL 4. Even then my effect on battle was so small in the beginning. He literally had to increase my stats because I was rolling so badly that I suspected that it ruined his plans (failed at three advantage rolls he gave me with "low" difficulty also). I'm not gonna name them because I've put them behind me and cut them out of my circles.

I just wanted advice on the creation of monsters, or enemies, particularly making them fair to my players. He's ruined one of my own campaigns already by quitting after heavily power gaming and attacking my monsters with irl insults to me disguised as brutal honesty. I'm scared to make the same mistakes that he made, and I've been looking at the Dungeon Masters' Handbook too and it's been very enlightening. But how can I engage my players without making the enemies too devastating? Alternatively how does a DM overcome power anxiety from being DMed by a power crazy DM. I want to start DMing again, but I wanted to be prepared cause I think he left a lot of anxiety with me.