Still attached to the past

I can't seem to move on from this girl. We started talking one day, and then after a while we stopped. Then I randomly get a call while at my cousins. Find out it's the girl. We start talking because her friend made the call, and even helped us meet up in person. We tried dating briefly but then she disappeared. I ended up dating the friend. We have been together for over a year now and I'm really happy with her. But I can't stop thinking about the first girl. I don't want to leave my girlfriend. But I care about the first girl. I got a secret sharing app on my phone called Whisper. I ended up finding her anonymously and we have been talking for almost a week, without her knowing it's me. I think we are growing closer, which I love. I love talking to her. I love being there for her. I have always thought, in the last couple years, that I can love more than one girl, treat them right, be there for each girl, and so forth. Which most anyone would say is polygamy. I don't subscribe to labeling myself as anything, and polygamy is not what I would call it for me. So... I guess I want thoughts. Help. I don't want to leave my girlfriend, I won't. But I can't seem to let go of the first girl. I care about her.