How do you put emotions aside in the interest of friendship?

SW

My friend invited me to her baby's christening but I don't know how to tell her I can't come.I helped her husband to do the baby shower but, the last time they had a christening at my church I cried throughout the whole christening.She is my good friend and she wants me to be the God mother .This is so hard sometimes. You have to put on a smile everyday but die inside..