How can I stop dwelling on the past?

Like what could have happened instead of what happened.

For example, I was able to participate in an exchange program on Japan, however the university closed due it covid and so my classes were online for half of the time I was there, even the closing ceremony was cancelled. Studying in Japan was a once in a lifetime thing, and I can't help but imagine how it could have been of the uni had been open ( it closed February), the people I could have met,etc