How come I can't get over this girl?

Why it's been a few months over this disastrous rejection during which i pleaded with her and begged to no avail (I know that's not the way to go about it). I can't stop getting over her. I think she'd be the perfect girl for me. When I talk to her I keep saying stupid **** that gets me in trouble online. She has graciously let me talk to her in a month. If you're here just to tell me what an idiot I was, don't bother, I already know but how can I get over her. It doesn't help that I'm nineteen, never had a girlfriend, and girls don't hook up with me at parties. Please I need some advice in help.