I am not sure that I exist, actually. I am all the writers that I have read, all the people that I have met, all the women that I have loved; all the cities I have visited.

The person speaking is saying that they feel they are composed of parts of the people and things that they have experienced in life. They are not sure if they actually exist in a traditional, physical sense, but instead consist of all the people and places that have been important to them. They could also be saying that their identity is rooted in the experiences that have shaped them.