Here's a link to a post about moving forward (not "moving on") with a new partner after the death of one's spouse. Spot on. Some favorite passages...

It can't be easy to be the man that came after.
I want you to know I don't wish things were different. I want you to know that I want you in our story. I want you to know that as difficult as life has been - every day with you tops my grateful list.

You don't replace the man that came before with the man that came after. You absorb the love, the lessons, and you morph into a deeper soul capable of rich love and special perspective.