Not everyone struggles with this, though. Given that it isn't an evenly-distributed problem, there is possibly a pattern between those who do struggle with it.
However, I have learnt to separate facts from knowledge. Reading books about marketing will mostly give you facts without context but if you to talk to people who do marketing or try to raise a blog to several thousand subscribers, you will gain a lot of knowledge.
Doing a lot of things, in my experience will give you more perspective and experience than anything else and frankly, there is enough time in the world to do that.
I'm the same as you, once again back in the career change cycle.
There's really no corporation out there who wants a generalist unless you're the CEO perhaps, but you have to be an expert to get there.
I'm pretty sure generalists were just meant to be merchants / small business owners but those opportunities have dwindled away in our modern economy.