Inadequate: lacking the quality or quantity required; insufficient for a purpose.

Incapable: unable to do or achieve (something). 

Incapable is the output. The what. 

Inadequate is the reason. The why. 

All of us are guilty of stopping at the what, whenever we fail. 

I am incapable  

I can’t do it 

I don’t have what it takes 

Only a few of us penetrate the why

Why am I incapable 

Why can’t I don’t it 

Why don’t I have have what it takes 

In my experience, surrounding yourself with circumstances and people that make you feel inadequate, will magically ensure that you don’t remain incapable over a prolonged period of time. 

But we rarely do 

Again, ask why!