How are you doing it? Paper or online, "Dear Diary" or "Dear myself", just go and write down whats on your mind or is there a "red threat" like specific questions that you answer every day? What are the positive effects in your opinion?
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It’s not exactly a diary but I keep a blessings book. You could call it a diary as I daily record the good things that happen to me. You’d be surprised about how many good things you can find on even the worst days.
Sometimes I will do a guided diary, where I find prompts to write about. I also write down songs, poems, speeches that I thought of. I don't do this every day, but if there was something that made me very happy or very sad, I will write down what happened.