IIN I can't keep a girlfriend because I keep thinking about her dad?

Trust me, not every dad thinks like this. Certainly my dad never did.

Anyway, a daughter isn't a man's property to give away. Both parents should naturally worry about who both their female AND male children end up with, but a lot of the time, the stereotype that people worry about is this overprotective dad who won't let anyone touch his daughter but high-fives his son for getting laid the first time. This is not only sexist to BOTH genders, but kind of sick. I always get a creepy vibe from these types. It's almost as if they've themselves laid a sexual claim over their daughters; like, I made her, I should have her. It's creepy.

Your own attitude toward yourself, and by extension other young men in general, is not healthy. The only question to ask yourself when you're trying to figure out if you deserve to be with someone is: does this person love me as much as I love them?

This girl is not her father nor her father's property. If he thinks he has any real clout in what his daughter does, he's sorely mistaken. The only two people whose opinions matter in a relationship in the end are the two people in it. Period.

Obviously, you do have self-worth issues to work on, but for now, just focus on how you feel about her, not on what her family might think of you. When you do learn to love, respect, and appreciate yourself (AND your gender) in all your qualities and flaws, you'll realize you DO deserve love and nobody should stand in the way of that.