Seeing a former good friend is really crushing my heart every time

I used to be good friends with this person (even had some feelings for them), but a little while ago it fell apart and we’re not that good of friends anymore. The problem is that i often see them due to my job and activities surrounding it and it hurts me every time because we became so distant and aren’t friends anymore. It also comes up in my mind often and just really makes me feel bad. Any advice on what to do?( Just stopping working together with them is not really an option)