There have been mixed marriages forever. Some how some way people find a way to make things work.

There are a lot of things to take into consideration before you even worry about the afterlife.

Will her family reject her if she marries out of her religion?
Will yours?

If you have children, how would you raise them?

Can you respect her beliefs even though they are different than yours? Can she still practice her faith after you marry or would you want her to convert or follow your religion?

You may want to find other people who have gone through this, especially Mormons who have left the church....they can probably give you good advice through their experience.