How does one stay content no matter what life brings?

One doesn't. And that's a good thing. Sometimes things need to change for the better before one can be content.

I think a constant state of contentment becomes numbness.

By knowing ,deep inside yourself, that you can and will over come anything

You don’t, you just have to learn to ride the waves.

Lobotomy

Some things are painful. We learn to live with painful events over time.

It's good to hyperfocus on good things that occur in one's life and to be thankful. Other areas, make a plan to improve being thankful for progress.