I sing at a lot of funerals, and it doesn't seem to get easier. Luckily, I rarely sing solo. Maybe that's good advice - if you're the type who's likely to break down, sing with others.
Last January, I went to the funeral of the 19-yr-old kid who used to live across the street, a kid I'd known since he was a baby. During the scripture readings, his classmates would read a couple of verses from the 23rd Psalm, and the choir and congregation would respond with a sung verse. It started beautifully, and then one of the girls broke down in tears while she was reading her verse. The only response the congregation could muster was a little squeak. It sounded awful, but maybe it didn't matter. What mattered was that there were 750 people feeling the same deep feeling at the same time.
-Joe Offer-