Welcome to our forum.
Just curious, why did you say this other girl would never go for it? Have you asked her? Sometimes people can surprise you. Also, it is sounding to me like you are envisioning a full-on triad with all three persons romantically involved with each other? If so, consider a V scenario where you have two romantic partners, but those two partners are merely platonic friends with each other. Just putting forth some of the possibilities here.
Alas, if it comes down to just "getting over" this person, I don't think there's any magical formula for doing that. Obviously if you can find distractions, that will help; also focusing on your primary relationship will help. But you can't have regular contact with someone and just wish away your feelings for them. Unless you truly know they're not a possibility? That's why I ask if you're *sure* this other girl wouldn't go for it. Maybe you need to *be* sure. In other words, ask and find out. Sure it is painful to face a rejection, but at the same time that might make a clean emotional break more possible for you.
You don't have to rush into any decisions, so take some time to read various threads on this site, and post your comments/questions. I'm sure you'll find a solution to your dilemma in due time.
Glad you could join us; hope you enjoy your time here.
Love means never having to say, "Put down that meat cleaver!"