I think it's entirely possible to be friends with an ex if the breakup was amicable and you just 'grew out of ' the relationship. I actually know many people who had horrible relationships and say they've become better friends since they're not together. It's all in your mindset. You can't worry about what the ex is thinking either. If your partner really loves you they're not thinking about that person, so don't waste your time wondering about whether they're going to try and pursue the relationship again. If THAT'S a legitimate concern, then you probably don't have that great of a relationship in the first place.
Live as if you were to die tomorrow. Learn as if you were to live forever. - Mohandas Gandhi