I want to be her dad?
Why do  you want to be her dad?
Because I have been in her life for a while?
It is the responsible thing to do if you have been in her life for a while and plan to be there for her for the rest of her life. It is something that people will admire that you are there for her being a good role model.
She's like my real daughter. I want to protect and guide her .
I'm glad that you love her like your daughter and you want to make it official. You definitely should set an example and be a positive role model in her life.