[syndacks]

>inevitable loss of ambition’s fulfillment

You are missing a key apostrophe there, "'", which grammatically means that ambition belongs to fulfillment.

The author is saying that as he ages, the fulfillment he once felt by being ambitious (e.g. writing books, starting a company, learning new things) goes away.

Does that make sense?

I think it's important to keep in mind that this isn't a "factual" piece as much as it is his reflection on aging. It might be true to him but not true to others.