I think the first and best step to getting things out – but not necessarily writing your life story for anyone else to read – is to buy a journal. Start writing about your experiences and painful memories. It will hurt, but the only to get past the pain is to go through it. The more it hurts, the better it will be in the long run to process the experience.
If you are not sure that your whole life path is worth attention, then a memoir is your best choice. It is focused on one particular event, relationship, place, or period of your life that has influenced your personality a lot. Don’t be afraid to express your personal feelings and ideas in your memoir, as it shouldn’t be simply a brief list of facts, but the reflection of your inner world.
What's your central conflict? What's the biggest obstacle life presented that took years to overcome or come to terms with? Maybe it's an illness you were diagnosed with at an early age, a relationship wrought with turmoil, a series of career setbacks, a goal you worked for decades to achieve, or any other number of things. Look to your favorite books and movies for more examples of conflicts.