why am I crying when I should be celebrating?

Crying... is it a stress relief? You've put in your time, suffered the harsh side effects, hoping they will fade away and eventually you can get back to normal. Or, is it the fear returning that although you're done with this part of the battle, you really don't know the outcome. Did you win? You don't know if you've beaten the enemy or he's just hiding and biding his time before showing up again.... months or years down the road. The worry is still there.
It feels like a recuperation is in order... a restful trip, a time away to rebuild my body and soul. Give me strength emotionally and physically, make me happy and whole. 
I pray this for all my comrades fighting cancer, for all the people I met at radiation, and for all the friends I know that are battling a lingering disease.  Find your strength. Find the little things that bring you joy. It's okay to cry... it releases yourself to be able to rebuild.