Thanks to all for your replies. It's good to know I'm not the only one feeling this way about losing a loved one. There's something special in a man's heart about his firstborn -- not that I don't love my other kids with all my heart. But it's that one that starts us thinking about how we fit into this whole big picture.
Somebody tried to tell me when my son died that it was probably merciful on God's part, maybe something terrible would have happened if he'd stayed here. Another friend put it better. He said, "As a Christian Dad, your most important job is making sure your loved ones all get to Heaven. You just had one go ahead earlier." It;s good to look forward to seeing them all there some day. Only through the Lord Jesus Christ.
Also, I know what you mean about not shedding a tear at the funeral, being glad they're out of suffering. That doesn't mean you don't feel anything either. It means you lived through that stretched-out grief with them.