Expresses emotions to particular person, usually ex-partner, but backs away when that person returns the sentiment.

Sociopathic behavior?
Antisocial behavior?
Manipulative behavior?
... or just scared of emotions.

Probably scared (surprised?) to be taken seriously. The recipient needs to change his/her manner of reacting.

More than likely scared, could be inexperienced in this social area. This is not strange and is very common, especially as a first experience, among young people, even adults if the feelings are intense enough.

I find it strange that your first 3 guesses are diagnoses. And the first one is really serious and ominous.
I hope that you can free yourself from the terms you find in a diagnostic manual.
Even 'manipulative' is wide open to interpretation, and gets overused.
A marionette is manipulated. Small children are manipulated.
Adults often claim they are. Are they? Or are they just not able to make decisions on their own? Easily swayed? Gluttons for punishment? Very insecure?

I wouldn't even begin to comment on that person's behavior! You have told us essentially nothing in a one liner.

Many people have attachment issues, it is really not uncommon. (both ways, too attached or afraid of commitment.