When I feel like shit I find that thinking about the possible worst thing that can happen to me (not related to the current thing that has me feeling like shit, just in general) then working from there helps a bit. Also, the worst outcome of the current situation. (yours seems to be that you lost your best friend)
The worst situation obviously is that person is never in your life again. Being able to accept that and seeing if there's any positive spin, that could help.
The worst situation obviously is that person is never in your life again. Being able to accept that and seeing if there's any positive spin, that could help.