I'm sorry to hear about your loss. I've never dealt with loss quite that close to me, but I feel like I can understand how you feel.
As Z Chopper said, funerals are a celebration of ones life. It's understood why a funeral would be a sad place, but there's no reason why you can't smile as well. Tell stories of the good times because thats what you want her to be remembered for. Don't be afraid to show a few tears, as its natural and emotions should never be held in.
It's not easy talking about death even though it is part of the very essence of life, but it is necessary. I think you'll find that when you stand up there and get comfortable, the words will come right out and you'll feel a lot better.
Just keep your head up high and know that you have people all over the world standing behind you.