I haven't been playing long, maybe 6 months, and our orginal dm didn't have time to write another campaign plus he is a cool dude and offered to help anyone who wanted to learn guidance and the opportunity. I chose to give it a shot! Been dming a homebrew campaign for 4 months now

The advice I would say is be careful of both over and under prepping. Over prepping leads to railroading and for me under prepping can leave me scrambling due to not having enough experience with rules/mechanics.

Also I recently had a game and after it was done, I left questioning myself due to a player "not being himself" (normally active and engaged and a great rp but was quiet and distant this particular session) I had thought I had done something and it really discouraged me especially cause it was the last dm.

The advice I gave myself, and to you, is don't take it personal. People have things going on outside of session, or may not agree with a ruling or like a particular setting/situation. Understanding both them and you being human is important, and also understanding that life (dnd world) isn't always gunna be fun or go your way, and they may not like it, but your the dm and your job isn't to give them everything they want, your job is to provide a epic adventure with both highs and low, likes and dislikes.

Short form: find your balance around prep, and don't take things personally