I think you've received a series of good advice so far. While I certainly see the wisdom in keeping a bad person out of our lives, I found Rowan's post about second chances particularly insightful. All in all though, it's your choice and your business how you deal with your father.
What to do with your father aside, you need to ask your girlfriend to accept that you will do what's right for you .....and to keep her thoughts to herself.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.