How can I fix this situation?

[Susan]

I see you are making a wise decision to try and fix a difficult situation. I’m approaching the situation as an adult of 3 grown children. You sound like a nice young man. I would like to pass on to you what might be going on in the fathers mind and world. That way you will be looking at the whole picture, seeing things from his point of view.
I would suggest starting with trying to find out why her father doesn't want you seeing her. There could be several reasons; many of which are in your power to change or do something about. Several things could be going on inside his mind:
He could have plans for her future that don't include a boyfriend.
He could be very religious and you aren't of his faith.
Could be he is old-fashioned and feels there is a certain protocol to see his daughter.
He could feel she/you or both of you are too young
Then again it could be that he doesn't want her to see anyone.
Or it could be something about you.
Some of the solutions could be: to then take it slower and become a friend of the family. Let your light shine and show the family your good qualities. Good things can come to those who wait. Also depending on how you feel about her, would you consider changing religions or at least find out about theirs and to respect their beliefs, etc?

Ok now is the hard part. Take a good look at yourself both inside and outside. Get honest opinions from teachers and older people. Then you will get a clearer picture of how other people see you. They can also help you answer questions like these:
Is there anything you can think of that would make her father feel you would not be a good choice for his daughter.
Would I let my sister or daughter (when you have one) date someone exactly like me?
Do I look like the responsible type? Do I have a job or help out at home? Or am I a gamer?
Do I look sloppy? Or am I respectful, have good values and goals in life?
How do I treat my family and others?
Do I look like the type that would use her and then dump her or use her just for sex?
What message do the cloths I wear send to others?

Your girlfriend and her mom might be able to shed some light on the matter for you and you might be able to make some changes that would make you more acceptable in his eyes.
In the not so long ago past, it used to be that a young man would have to ask permission to date or see a young lady and it was for the purpose of marriage. So the young suitor would try to impress the parents by the way he dressed and acted. They still are this way in some countries.
There is a lot to say as to why he is acting the way he is. So you need to try your best to find out.
It takes maturity to think outside the box at the bigger picture. In the long run it will be worth it, because I'm sure you want to be with her for a long time and family comes with the package.
I hope this will help you in finding a solution.