Great post that resonated a lot with me, because I'm exactly the same. All comes down to not thinking you're good enough.
This is not a sob story and I'm not looking for any sympathy here, but as a child my Dad always told me I'd never amount to anything and everyone elses kids were so much better than us. Then it kicks in all by itself on auto pilot. You know the old story - If you're told something enough times you believe it, and so it did with me. I've gone through life thinking I was worthless, despite a University Degree and a good career.
Funny thing is, myself and my brothers have surpassed "everyone elses kids" in life. Ha !!
The stupid thing is, I know deep down that I'm good enough, but it never stops me from believing I'm not. Go figure that one out ! I can't shake that belief that was set in stone.