I can see the trees, but can't see the forest

I hope someone can help me identify this learning disorder. It's hard to explain, but let's say I can see the trees but can't see the forest.

Basically, I have a problem putting the pieces of the puzzle together. I can grasp individual topics fine, but making sense of them together and combining them is a huge problem for me. I used to mention it to my doctors, but they just said I wasn't trying hard enough. And I took it that way.

But now I'm in my 40s, and it just keeps on repeating in every single thing I try to do. I tried doing many things in life, and the pattern is always the same, I can learn individual topics fine, even the hard ones, but putting all that together - no chance.

And now it's starting to pose a problem for me, I have a decent amount of knowledge in a couple of fields, but I can never put it to good use, and I end up losing jobs regularly. Employers get pretty enthusiastic during the interview, and I usually get the job easily, once I present my ideas, but once I start working for them, I never live up to their expectations, precisely because I can't develop those ideas fully.