Richey,
I used to have a really tight mind, which made my life pretty uneasy. I was seen as a really serious guy who tried too hard to be funny. That constant trying to be witty is really tiring, what happens is that we try to be 'somebody else' for other people. This is a major stress.
Are you trying too hard? If you are, you are more than likely experiencing a lot of anxiety. To lighten things up, your mind needs to be more still, more peaceful, it seems like your thoughts are scattered and confused. Everyone is funny and has funny things to say, when they become too interested in what others think of them, instead of being able to free flow and play, they become self absorbed and anxious, quite unpleasant. So instead of having fun, it seems like a test thats not real and the test is on you to figure out whats going on.
All of this stems from 'what others think of you' - which is why many really funny people experience lots of anxiety also because they are solely dependent on what other people think. Comedians often become depressed because of this. Essentially, you need to remember that your happiness depends entirely without an exception on your own mental peace or mental actions.
Forget trying to be funny, forget about coming across as boring, when you relax you will see that you become as light as a feather.
Jack