That vital part of learning. Many skip it. We know how to receive instructions. We know how to apply. We hardly reflect.
Writing to your younger self puts you in that reflective state.
You view circumstances and seasons of your life void of extreme emotions - fear or excitement. You just see those past situations clearly. As a life coach would.
At that point you are able to "mentor" yourself.
Sorry, your younger self.
The truth is, the lesson you are passing to your younger self is very much needed by your present day self. You are able to see silly mistakes you thought you have dropped. You also come to see and appreciate how much you have grown as person. You would literally track your maturity.
You can write this as series,with focus on various aspects of your life - family, relationships, career, money, health, character. Advise your 10 or 20 year old self how to handle money; how to avoid the career mistakes you made and so on.
You will be glad you did.
@Keni describes the experience as introspective and healing.
I leave you with this statement by Rick Warren.
Clarity is often hindsight.