First of all, you're not an idiot. You wanted to pursue him. You went with your idea. That's an achievement. The not working out part? psssh Those happen. You wouldn't have any way to know without giving it a try and you did. Be proud of that.
Maybe he thought he could do poly. Maybe he even wanted to. I don't know him so i can only speculate on his thoughts. Maybe he is a lemon. Lemon's happen too.
Keep your wits about you, process your feelings, and go find a tangerine or a huckleberry or even a pineapple. There's all kinds of wonderful fruits and vegetables out in the world