Am I a downer?

Why is it that I feel I used to be a much happier, more joyful person than I am today? What made the change and why? And especially, how can I truly feel and show others God's joy again? Don't get me wrong: I get that I'm totally blessed. I know that. I acknowledge it. I try to revel in the gloriousness of it. And yet, somehow, I still feel like I'm not actually joyful. Deep down, there's a sadness...or a resentment...or a shame. I'm not really sure which, but it's very wrong and I don't want to be that type of person.