felt like a square peg in a round hole?

As this image shows, the saying refers to something or someone trying to fit in where they don't fit. In your quote above, the words that match this phrase are "I was never cut out for this."

The person is saying he never fit with the world of finance. A square peg doesn't fit into in round hole very well.

Ha!

Felt like a square peg in a round hole is an idiom or a proverb indicating that the person in question doesn't fit in her or his surroundings.

Perhaps Sadowsky doesn't like the stresses, her or his coworkers, the job itself, the managers, or the corporate lifestyle. Use of the adage has the person acknowledge her or his social incompatibility to the banker's lifestyle and responsibilities.

The image provoked by the wording is meant to indicate that no matter how hard you try to hammer the square into the round hole, that it won't fit.

Got it.

Thank you guys.