I have a major problem with emotions as well. I have been told by many people that I am rather cold and apathetic. It took me a while to notice for myself, but over time I have come to realize that it is true; I am unable to effectively show emotion.
I have seemingly permanent deadpan expression - that borders on a sort of scowl - on my face. Many people find it very misleading and it turns them away. I'm almost certain that it plays a huge part in my not being able to make friends or even look people in the eye. Because of this, I have become incredibly misunderstood, and a lot of people just don't "get
My odd facial expression appears to have originated from when I was younger; I'd frown a lot
and I looked down at the ground all the time. I guess you could say it got stuck that way.
The rare occasions where I have actually smiled were much like the OP's. I would feel like an idiot or a loser for having..."nearly enjoyed myself"...and done something positive. It feels as if my brain has an aversion to positive experiences. I'd always go into a depressive state during those times, and I'd spend my days and nights - without sleep - thinking about how "stupid" I must have looked in said situations.
My apathetic tendencies might be loosely connected to the autism spectrum, as I was evaluated for Aspergers at a young age, but there are many conflicting factors that almost rule it out as being a possible cause. What is more likely is that I have somehow subconsciously activated a sort of defense mechanism to help cope with my social anxiety and depression. Unfortunately, the true reason as to why I am unable to show emotion remains unknown.