Here's a question for the hive mind

So, one of the things I have trouble with, sometimes, is believing that I exist. (Yes, I know. That sounds stupid. But it's how I am.) There's a piece of me that finds it reasonably clear the the external world is real, but not that I am. Certain things/events/behaviours make me wonder if I am, in fact, just a fiction.
The marquis, on the other hand, is pretty sure he exists, but isn't always so sure about the external world.
What about you lot out there? Do you believe you're real? If so, why and how? Conversely, do you believe that the external world is real, and if so, why and how?
(I believe in the external world because that's clearly where all the power and control and entitlement comes from, and where all the important things are.)