...We must have been separated at birth. That almost totally describes me when I'm around people

I never seem to feel like talking to people, usually strangers/ people I'm not confortable with. When they do talk to me it's like my mind's only half-working and I can't think of anything to say that will keep the conversation going. And when the conversation ends my mind starts working again and I think of things I could've said to make it last longer.
I don't know the answer exactly. Could be shyness, which could go away by having more experience talking to people. Have more confidence in your abilities. Or act like you're confident, and eventually you will be confident. Or maybe you're a loner by nature who doesn't care much for establishing relations with others? Well I don't know you, but for me at least, it's a combination of all. I used to not worry or care whether or not I was by myself for a little while, then I noticed how everyone places such an importance on friendship, and how happy everyone seemed to be in their big group of friends and I started wondering if I could be happier that way. Buut with my shyness and fear of making myself look stupid, I have a hard time starting casual conversations with people. When I do start one, it's usually not meant for a long chat. I usually prefer conversations on certain topics, and friends, famiy, or people I'm comfortable around, starting conversations with me
Sorry i spent a lot of time talking about myself ^^' More than you needed to know. If you really don't mind being by yourself most of the time or don't care about what other people think of you, you shouldn't worry. But if you want to improve relations with others, you can go talk to a psychiatrist or join a group or club with similar interests as you and start something from there