How do I let go of this girl, she's not a good person, help?

[Aj]

So here's the thing, there's this girl that likes to gossip a whole lot about people she hates, about where they were, who they were with, you name it. I used to be friends with this girl over 5 years ago and actually started to get a small crush on her. It was kind of funny, but she also liked me, and admitted it, but I was very shy back then and it caught me off guard and I turned her down. Ever since then, she's gossiped about me, constantly put the girls I've dated down, gave me garbage, and would try to get others to join in. She was a huge time jerk, and I thought she would move on because I got word that she got engaged, but she kept up. During Valentine's Day, I took out my girlfriend to a restaurant that she happened to be also, and for some reason didn't notice my girlfriend and just stared at me the whole time, and left with her brother saying something stupid to me. I heard from a mutual friend of both of ours that if I was to apologize, she'd give me another shot, the thing is I know better and know that she's not a good girl, the only thing I hate is I seem to be infatuated with the past, about her liking me in the past, not necessarily who she is now. I'm very much in love with my current girlfriend who seems to be inseparable with me, and I wish to give more to her and let go of this stupid memory that took place 5 years ago when I still attended high school. Any advice?
To make this more clear, I'm in my 20s now, so I'm mentioning a friendship I used to have with her when I was a teenager.