How do we build inner strength? Do we push ourselves to the limits? Do we attempt really difficult things? Do we take enormous risks? Well, maybe. But I have a different take on this.
Thinking
Switch off the incessant damaging narratives in your head. These loopy narratives are often angry, cynical or fearful. Cultivate inner silence. This is difficult. It takes years. Start by noticing the repetitive stories and detaching from them.
Fear
Never (ever) act from fear.
Notice, I did not say, do not feel fear. Trying not to feel fear is futile. Rather, notice fear. Feel it. But never act from that state. This is not to say that we should not be prudent. Nuance matters.
Anger
It is rarely good to act out of anger. Again, nuance matters.
Ask yourself this, Is my anger enforcing a strong and necessary boundary? Or is my anger rooted in wounding? Action based in the former is (probably) a good idea. Action based in the latter, not.
To move past the anger, feel it. Make friends with it. But do not act from it.
Grief
Feel your repressed grief. This is the only way to move past it. Cry. Connecting with nature can make this process more gentle. Allow whatever is there to build and release. Cry.
These are foundational things to build inner strength.