Perhaps Vito was to some degree as manipulative as Michael when he manipulates him into taking over the olive oil business. And Michael inherits all the mess and unpayed checks - the Barzini coup, killing Carlo, etc.
After Michael had come back from Sicily, he wanted to do everything right (= right in the eyes of his father). And that's what he promised his father: "I can handle it."
Michael is honest to his mother and maybe she's one person he's not trying to manipulate. He asks her: "But by being strong for his family could he [Vito] lose it?" Of course, he's talking about himself. He's trying so hard to protect his family but on his way his heart became cold. He couldn't really love his family anymore, so he lost it.