If you developed a serious relationship with an SO and learned that he/she had a previous SO who's relationship ended on good terms, job relocation, family necessity, etc.....how would you feel about later contact/communication between the two of them?

What would it take for you to know that YOU are number 1? Could you ever?
What would you do if you felt there were things not shared with you?...Yet, at least?
What if you were number 1 ONLY because he/she were unattainable?
What if their relationship were closer than siblings?
Could you ever feel secure if THEY maintained contact?.....Would it be right to ask them not to?
Would you feel like a fifth wheel?

Scary, huh?....I think it is....But I'd hate to lose contact with someone who meant a lot to me in the past but it didn't work out for reasons out of everyone's control.