Angst and the story of my life

Why is it that I'm always letting myself get into a twist about things? I know I put lots of pressure on myself to do the "right" thing to make the "right" decision and do the best that I can. Why though? How do I become one of those people who can just ride the wave of life and enjoy it for what it is?
Taking things at face value sounds like such a wonderful thing to be able to do. So does living in the moment. I'm perpetually fretting or feeling anxious about something I said or did, or worrying about what's going to happen later, tomorrow, next week, next month. I know that there is very little value in that, and I'm letting the wonderful things about right now whiz on by me by doing that, but I can't seem to help myself.
I struggle to be positive, open and in the moment, and then beat myself up when I'm unable to do that. It's a vicious cycle that I can't figure out how to break.