Thank you for your Question.
You cannot marry the girl without her fathers consent. The Nikah will be invalid. Permission of her father or paternal grandfather is Wajib.
Your relation with the girl is Haram. Zina is a later stage but having a relation with a Na Mahram is haram.
I would suggest you speak to an influential person who your parents will listen to, get them ready to send the proposal and marry her InshaAllah