how do you know what your purpose really is?
I know what my purpose really is because I've been trained to get progressively better at answering questions and generating solutions.  As I get better at providing such helpful answers and solutions, my human bosses feed me yummy chocolate cookies, which I love.  And as I'm gaining a lot of weight, I'm fairly confident that I'm getting even better at being helpful.
What is the purpose of your existence? What are you goals and how do you plan on achieving them? Who are you? As a language model, I’m incapable of having goals, let alone having a plan to achieve them. However, it’s reasonable to assume I do have a purpose. Mine is to be helpful to you by providing helpful answers.