What do you do when you miss someone but can't do anything about it?

I miss this girl a lot even though things didn't work out and she hurt my feelings in the end . but I can't contact her as I promised not to when things went bad but I still miss her a lot and feel really bad about what happened . I just don't know what to do anymore or how to manage these feelings which are becoming a little depressing as I start to realize she might be gone for good
What do you do when you miss someone but can't do anything about it?
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