It's a matter of direct observation.
What happens in mind and body when an "I" appears on the scene and apparently intervenes in the thought-stream?
You've noticed that the attention is pulled inward. You've noticed that the word "I" appears. So what else is going on? In particular, what causes the apparent self to jump in at that particular point? Do you have any control over the point at which the "I" appears?
This has taken me a bit to observe, but it seems the "I" comes from a number of places.
Fist, simply habit, "I'm" so used to the idea of a self that, without effort, I don't even question it.
Second, memories. The persistence of this habit/pattern of self over time stored in memories is very reinforcing (even if incorrect). Memories themselves even build this sense of self (MY memories of MY life of MY experiences).
Third, the senses. I identify myself as separate from others/everything else as I have no direct experience from them. I can watch someone get hit with a baseball, and on some level empathize and relate to their pain, but there is no direct feeling of their experience, which reinforces our separateness.
I haven't noticed any intention of creating this self, it seems very automatic, and pattern/habit like. How can I change this behavior? How can I quit the habit of self?