I honestly believe you can never tell if a relationship is going to last. In my own marriage which is going on 14 years I don't think of it as 'I'm going to be with this person forever.' Instead I think of more like 'I'll probably be with this person for the next six weeks. Then I'll re-evaluate.'
The secret is not to give up hope. It's very hard not to because if you're really doing something worthwhile I think you will be pushed to the brink of hopelessness before you come through the other side.
