How do you get over someone that didn’t make a mistake?

How do you get over someone that didn’t make a mistake? When you’re the one that screwed up, and now they’ve moved on and are happy with someone else? I want to be happy they are happy, but the fact that I will probably never be with her again, or hold her, or see her smile, or rest her head under my chin is the most terrifying thing I have ever imagined.