Whether it was a short time, or a long time. Whether its the 1st time, 3rd time or 20th it doesn't matter much. It's how much that relationship, that person affected us that tends to dictate the severity of the hurt. I suppose some people have an easier time letting go than some. When those of us that do get hung up on these things people will often say, "They just don't understand how relationships work". I think that's quite unfair. A persona can understand how things work and still have feelings of devastation. The mind and heart don't always go along in sync or agree.
When it comes to those of us on the spectrum, no matter what our 'norm' may be I tend to think that when we do feel, we very well may feel more than average. This is where we end up shutting down, melting down, are unable to transition etc. So if there is a time that we connect - something we normally have a hard time with, whether we intend to or not we may get more emotionally wrapped in that person and so when they leave it leaves us completely out of sorts.(this is putting it mildly in some cases.) That isn't going to be everyone, but I suspect that its how it probably feels for you. It's how it's felt for me; just watching as they hold your heart and then let it smash to the floor like nothing.
I'm sorry you're hurting.
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"Inside the heart of each and every one of us there is a longing to be understood by someone who really cares. When a person is understood, he or she can put up with almost anything in the world."