Take comfort in the fact that the majority of us are the same deep down (well, I know I am the same as you and others will be too).
Most people just have a good way of hiding behind a 'face', choosing to mask it rather than address it - I know it's a cliche, but it's true. My mum always used to say to me: "You think that everybody you see in the street is happy just because they are smiling?" It's a fallacy, and I bet others have seen you laugh at some point and wanted to be you.
Rather than trying to become something you're not (i.e this lively, postive can-do go-getter), focus on what you are already, your good points. I bet you are caring and a good person, and I bet you have a talent or two as well.
Try not to compare yourself to others too often, we all do this and this can bring down even the happiest seeming person. Do you have a hobby that makes you feel good? If you don't - FORCE yourself to take one up that you think you'll enjoy.