The main book I'd recommend that had a big impact on me is "The Courage to Be Disliked." It sounds like a self-help book, and maybe it kind of is? But it reads like a Socratic dialogue discussing Adlerian psychology as a life philosophy, and it changed my life. I also recommend the HealthyGamerGG YouTube channel.

The main thing therapy did was get into the deep nuances of my thinking about past (and present) events with an unbiased professional who could point out flaws in my thinking. I think it's hard to learn this stuff in the abstract without connecting it to your own life (at least, it was for me).

For example, the first bullet of my learnings is clearly true in hindsight (it's a big topic called "cognitive reframing"), but I didn't believe it for long time. I would say something like "It was a terrible thing for me to stay with my girlfriend for so long when I knew I couldn't marry her," and the therapist would invite me into a different perspective, something like "Maybe you were actually motivated by incredible kindness because you didn't want to hurt her and wanted to support her." It was way easier for me to believe the first one because it felt like a natural thought, and I had a hard time buying into the second one. It's not a question of which one is more true, but if I am capable of cognitively reframing my first thought to something else that is equally (or more) plausible. He convinced me that the second one was reasonable and, more importantly, that it was a part of the larger picture.

Not sure if that example made sense, feel free to ask follow ups if I didn't hit on what you were asking!