If you change girlfriends every two months, you will have several former girlfriends, but only one ex-girlfriend. The word ex should be used with the immediate past incumbent. In the other words, your ex-girlfriend is the one with whom you just broke up. All other girls you dated in the past are your former girlfriends.
- He still maintains good relations with his former girlfriends.
- I ran into my ex-boyfriend yesterday and it was not a pleasant experience.