His decision to pursue another girl has nothing to do with you as a person.

Practically speaking it could have been for any number of reasons: location, circumstances, he feels like his personality clicks better with her. None of these things make her better than you - nor do they make you worse or the second choice. At the end of the day each person is looking for someone that checks as many boxes as possible. That's why relationships are so hard... You've gotta find that someone where both of you wouldn't ever want to search for anything else.

In order to best prepare yourself for this situation I would focus on myself - work on you and improve how you view yourself. This will have positive impacts in more areas than just your love life.