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Do you want to get better at a certain skill? Read about it. Be more meditative? Read books that explain that in detail. Want to be more productive? Spontaneous? Outgoing? Confident? All these topic areas are covered by books upon books that you can study--and by reading about it, it'll always stay top of mind.
A mentor can be anyone from a peer who knows something you don't, and you want to learn, all the way up to someone vastly more experienced who is willing to take you under their wing (in exchange for your working in some way for or with them). Mentorship is by far the fastest path of learning.
If you really want to take self-development seriously (and not just, you know, talk about it), you need to be constantly aware of how you can improve. And the only way to know how to improve is if you reflect and ask yourself where and how you still need some work.
Explanation: