the main thing I remember changing my way of thinking was the notion that many of our goals in life are simply conventions passed down uncritically (e.g. by parents , society, etc), which if you examine you might realise you are trying to conform to, but don't actually align with your core values, and may even be making you unhappy for no good reason.

e.g. in some cultures it's the whole "you should be a doctor or a lawyer", which has led to people who would have been happy artists becoming miserable doctors and lawyers instead.