Epiphany...

I have finally concluded the biggest reason for my jealousy.
It is her desire for secrecy. We had a relationship for four months. She, for reasons I cannot fathom, remained firm in her want to keep this relationship a secret to everyone.
When I asked her why nobody could be told, she said it was because of the risk that she would be relocated to another institution. This, I could understand. But she also refused to reveal it to anyone, even those that could in no way affect her circumstances. This made me paranoid of her intentions. It was not that big a deal for me until she started spending an inordinate amount of time with her ex, yet refused to reveal her current relationship status to him.
He is convinced that she is with him again. When I begged her to tell him, she agreed. Several days went without her telling him. Her excuse was that no opportunity had risen, this, of course, was a lie. He had no doubt asked her if she had a relationship with me, and she answered in the negative.
When I played along with his lie that they were together, I used a lie as an excuse for making an conversation.
My goal was to make him reveal his lie, but I failed miserably. The lie I used at that specific occasion, was that she had kept their relationship a secret to me. I expect him to hardly react, and maybe lie further by saying that he was aware of this. He did not react this way, instead he seemed very angry with her for keeping their relationship secret.
So far, his lie has been more convincing than her truth, just by his unconcious behavior. His intellegence is insufficient to lie so expertly, but I trust her, until she gives me further reason not to.
All in all, the source of my jealousy is her secrecy.