sorry to hear that McKinsey didn't work out. But keep your spirits up, something else will work out.
Regarding your question: The thing about conceptual and outside of the box thinking is, that it's very hard to "learn" in a sense of "if I just do 5 cases of this, and 10 cases of that, and memorize the key points of these 3 books, I'll be good at it". Quite the contrary, too much cookie-cutter work can actually hurt.
So the best advice that I can come up with: Try to broaden your scope. Read things about philosophy, social sciences, biology. Read biographies of interesting people - Abe Lincoln, Mahatma Gandhi, Nelson Mandela, Elon Musk, Bill Clinton, Barack Obama, Mao, Hitler, Leonardo Da Vinci, Hannah Arendt, Augustus, Queen Elizabeth I, Howard Hughes, Ben Franklin, ...
That's obviously quite a reading list. If you want to stay closer to business and economics, try some books that establish fundamental concepts, like
- "Competitive Strategy" by Michael Porter
- The Innovator's Dilamma by Clayton Kristensen
- Zero to One by Peter Thiel
- Against the Gods and Capital Ideas by Peter Bernstein
- Principles, by Ray Dalio
- Why Nations Fail, by D. Arcemoglu
Obviously, none of these will give you the answer. But hopefully, they will help you shape your thinking, help you spot patterns and openings, and just make you more creative.
Also, to help creativity and out of the box thinking you can actually try some mental experiments, like
- If you thnk the solution to a problem is obvious, try to ask yourself what would happen if you (or the company) did the exact opposite
- Regarding product development / customer service etc. ask yourself: "What would a 10-star experience look like?" (look up Brian Chesky on that one)
- On Market Entry / Strategy cases, try to flesh out a number of competitive apporaches (price leadership, differentiation, focus...)
- If you look at markets, try to ask yourself how these could be disrupted, or how the value chain could be reconfigured? Does vertical integration make sense? If so why? Where are economies of scale prevalent, where maybe not?
I'm not sure if all of this helps you, I guess the key message I want to convey is this: Challenge your thinking to look beyond the "obvious", "correct" or "learned" solution. Most of the time, you probably get stuck in the mud, but sometimes you strike gold.