As the title suggests, whether it's written on paper with a pen into your book of mirrors or typed out in a document or email, write to yourself. You don't actually need to send it. The act of just writing the letter can help to analyse or uncover feelings from within the subconscious bringing them to the surface. Which can then be worked through. You could also write a letter to someone else to express your thoughts and help to discover your true feelings. Then destroying the letter afterwards.
Personally, I have written letters to a passed relative and then to send it I put the letter into the fire of my chimney. Knowing my loved ones witnessed me writing the letter and aware of it's contents.
Another way that I do the above exercise is each and every year on a specific day of the year, I write a letter to myself from myself. Within the first half I write about my impressions of the year that just past. About the hardships or good times had. Then within the second half I described my desires and thoughts for myself in the next year to come. About my dreams and goals that I hope to achieve. When completed I place the letter in a special small box that is stored and saved until the day comes again a year later when I open the box when I read my letter to myself and again respond in kind.
Becoming an annual tradition to reflect and wish for myself, it's interesting to compare what I have hoped for and how things turn out afterall. All the letters are kept in the box and I can look through the letters on that one day of the year when I reflect on my growth and see my progressions and lessons in my life, etc. The day of the year to do this exercise annually could be any date, like your birthday or new years or any anniversary, etc.