Arsenic, I have to agree with Star. He needs to learn/understand that even if you get cross sometimes, you are still his Mum and safe person and love him unconditionally - nothing will ever change that.
He also needs to know that you aren't perfect and can be emotional or make bad choices/mistakes sometimes and that that's ok - it doesn't change how you feel about him.
I'm not exactly known for my calm demeanour, but ds knows that I love him no matter what. We've read books to that effect and talked about it regularly since he was old enough to understand. He now gets it that even if I shout and/or get angry sometimes, that doesn't mean I don't love him, it just means I'm human. He knows that I'll always love him and put him first no matter what. Conversely, he also knows that no matter how much he shouts at me and has meltdowns, it will never stop me loving him. Two sides of the same coin.
Your ds probably moved on from it so quickly precisely because he knows it too. You're his Mum and you love him, to him that will be an indisputable fact.
Again, as Star said, all children learn to deal with disharmony through interaction with their parents - we're the safest people to do this with, precisely because they do know we'll love them no matter what. If they didn't, we wouldn't get it coming the other way with toddler tantrums and teenage strops.
But then, you know all that already, you are just having an attack of the Mummy guilts. We all get them sometimes.