Well, her anniversary, is about her, not you. As difficult as it must be for you, be happy that she will not spend the rest of her days (and I hope there are many) alone, with no partner with her. I'm sure you would not want her to grow older, alone, with no one to talk to.
Try not to think about the "other stuff." Seems to me like that will only make you more uncomfortable.
Maybe give yourself a pep-talk before getting on the phone. As intuitive as women are, she'll hear the disappointment in your voice if you're not genuine in your well wishes.
Good luck, I hope it goes well