I'm Not a Child
Since I was a child, I've thought I could change the world easily. I haven't done any important thing so far. Now I've understood I understand that I'm an ordinary person with a lot of limited abilities.
Now I feel I'm not a child anymore.
This is also correct:
I have come to understand that I am an ordinary person . . .
I'm not a child
Since I was a child, I've though (have been thinking that) I could change the world easily. I haven't done any important thing so far. Now I've understood that I'm an ordinary person with a lot of limited abilities.
Now I feel (that) I'm not a child anymore.