How to get over/stop obsessing over a friend who I used to see as a potential partner?

Long story short: I (23 M) in a long distance relationship with my girlfriend (25 F) and in my area I have this friend (21 F) and we grew really close a couple years ago, to the point where she sent me nudes. I asked her out during that time, but she told me she wasn't interested in a relationship so I moved on. For some reason I still obsess over her from time to time and it's really annoying. I know she's not into me and I should only have eyes for my girlfriend, but I can't help but think about her every so often. How do I get over her?

How to get over/stop obsessing over a friend who I used to see as a potential partner?
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