Agree with pps...sorry you're hurting, but please dont make a bigger thing of this than it needs to be. Have a cry and moan then move on.
If there was this crush for a long time, its limerance or something like that, if you've not actually dated!
I've been the "hot youngster" turned down for older women before. Think about Charles, Camilla, and Diana! You just never know what the dynamic is.
I'm sure you've turned down or disappointed perfectly attractive men before, as someone else happened to catch your eye.
I also think everyone goes through social phases where they feel attractive and then unattractive.
Maybe this is a sign you need to slowly broaden your social circles, hanging around in a small group with someone you have a big crush on can't be good!
Just take time and enjoy meeting people and civilly engaging with them without stressing about whether or not they see you as dateable.
Your crush may have been one of those guys who deliberately sends physical signals and hints and makes eye contact (ego boost for him) which is why you thought there was something there?
There's plenty of guys like that. All timewasters and all trouble.
I worked in a supermarket when studying and there was a (conventionally handsome) doctor who used to come through in the evenings
He would hold lingering glances with all the women, focus his attention 100% on them and occasionally drop into conversation how he'd just come from work as a doctor, did he mention he'd been treating ill patients in his clinic...
(.Of course he was married)
I thought he was hilariously obvious and slaggy, but it was clearly an "act" to get women thinking there was something more.
Beware the subtle signals guy!