This is something I posted before: 

Strength is when someone faces their fears. When you do something necessary, even when you thought it would annihilate you. It is when they grit their teeth in the face of everything terrifying and difficult, and hold their ground. Or even step forward, through the fear and tearing anxiety. 

Let me try again:

Strength is when you do the right thing for yourself, even though you know it will be very painful. Even if you don't know whether what comes next will be better, or worse. And you are scared.

Strength is doing this hard thing, this difficult thing, and allowing yourself to feel all the difficult emotions that come with it. The pain, fear, guilt, regret.