Former girlfriend. Ex-girlfriend. They both mean the same, but do they? Well, they pack a different punch, emotionally, so use former or ex based on what you wish to communicate about the subject and his or her relationship or standing to the thing in question.
For instance, you would use “former” to indicate a relationship that is in the past but where there is no animosity, loss of position or negative reserves to communicate. And where a relationship, in the sense of still possessing the skills or honor of the position once held, continues. You are a former scientist, or he is a former Marine, for instance, since once a Marine, always a Marine.
“He is a former boyfriend” or “She is my former spouse” is suitable to suggest a person whom you have no hard feelings towards. Or when you want to be Switzerland about the situation, and don’t want to let on how you feel about the person or relationship one way or another.
Ex on the other hand, sounds closely like “axe,” which is precisely the instrument that you may have thought about employing in relation to this person on more than one occasion. If that’s the case, he is indeed an ex-boyfriend, she is most certainly an ex-roommate, and that gnome is an ex-employee.
In other words, axe the ex is a good way to remember this rule, and refer to any ex-person, place or thing that is nicely tucked in the rearview mirror of your life. Or, you could always just say, moodily and mysteriously while gazing into the distance, “He is someone from the past.” Ah.