Well personally, I've always had a slight issue displaying emotions, and even processing them internally. At most funerals, I have not cried until much later when I was alone. I don't believe this is anything to be ashamed of, I just think that everyone processes traumatic events at a different rate. My "negative energy" if you will, is similar to how a capacitor works (if you are familiar with electronic components). It charges up, charges up, keeps charging until it can't and then it releases all at once. With practice and a bit of therapy, I believe anyone can learn to live with this sort of thing. Its just important to be alone at some point so that nobody witnesses the initial break (in my case at least).
I do believe it is important though, as you said, to remember that a funeral in most cases should be a celebration of life instead of a mournful gathering.