The feeling where you thought you found the mirror to reflect what you are; where the one you thought was the reflection of your mirror. 

But you merely overrated that person.

That person could not handle your Ego.

That person was too sentimental; not autistic enough to be the ideal. 

Alternatively, heartbreak is when you've fixated an ideal upon another person: and they either do not come to terms with matching that ideal [and vice versa]; or you were just too delusional to see that person could only see you as a friend - due to your conversations not being so rapid in contradicting each other in all forms of rhetoric - playfully and consistantly. 

I cannot descibe this feeling, for I am always the arse; I can only play with logos [or their passion I should say] what the other person must have felt.