I guess: Have you seen the movie "Jaws"？That is a very famous horror movie directed by Steven Spielberg. The movie is about how a Great white shark hunt and kill people. So I think "He was my jaw" means "He was my nightmare".

This doesn't mean anything. It makes no sense. Do you think that you may have misheard "He was my joy"?

Do you perhaps mean, "He was my backbone"? If so that means that he supported you through all the hardships and he was the one you leaned on when things were the hardest. If you do not mean that, then I have to agree with the others that I have never heard that phrase before.

I am also a native English speaker, and I have never heard this before.

As a native English speaker, I've never heard that phrase in my entire life.