How to be more care-free?

I have been told i’m like a third wheel in friend groups, and i get where their coming from. They’re all extroverts, and i’m not. I don’t even know how we became friends in the first place. Before i say anything, i always think it over in my head, and then after i say it, i think about how it sounded. But most of my friends can just come up with a witty/funny response in no time.

I don’t know what to do, so any advice is appreciated.