How do you move on if she chose another guy over you?

I was in a long-distance relationship. We used to be childhood friends and we were each other's first crushes. Without going into details, she ended up breaking up with me and although it ended on good terms, we don't speak to each other anymore, as she thought it was not right of us to do so (and I agreed).

I learned later that she was actually in another relationship when she was with me. I asked her about, she admitted that it was true. Basically, she was in love with both of us and didn't know which one to choose. I don't blame her at all. I know her well enough to know that she must feel terrible. And it's not her fault she couldn't choose me.

But it kills me to know that she chose the other guy. We were together for 12 years (since the age of 13) and I still can't imagine my life without her. It's been two months now, but I feel as bad as the first day. I never had to deal with such emotional turmoil in my life. I don't really know what to do, and if you could help out, that would be great.
How do you move on if she chose another guy over you?
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