There are many good reasons to consider writing a diary. Amon other things, it will help you get perspective and new insights. It also improves your access to your inner creativity and problem-solving. There are studies showing that writing also about tough experiences and negative feelings can increase wellbeing and help you heal, se later posts for specific exercises. Writing a diary by hand is also an important way to get to know yourself, your preferences and needs.
Try to write 3 pages in a normal size journal by hand for one month and you will most likely feel that you have reconnected to yourself on a deeper level. You will have the best effect if you write in the morning immediately after waking up since you then are the closest to you subconsciousness/dream world and you will be more likely to write about your feelings, dreams and hopes, instead of reporting you day and related incidents.
Also, as you write, do not censor yourself, ignore critical comments from your inner voice, grammar etc. Just keep going without lifting your pen. Always finish the exercise by naming 3 things you are proud of and 3 things you are grateful of. Common benefits of the latter are better sleep, fewer symptoms and more experiences of happiness, harmony and joy. More about the importance of gratitude in a later post.