Non-acceptance results in suffering.
But some things are simply totally unacceptable!
You will never be able to fully accept anything that you perceive as totally unacceptable. 
Stop trying - it is impossible.
But you can accept that it is as it is.
When we say to ourselves : "This is unacceptable!" - we create suffering.
Instead we can learn to say : "This is awful, but it is as it is".
This immediately calms down our reactivity.
It keeps us rational and open to finding ways to move forward.
If we can do something about whatever is unacceptable - we do it.
If we can't - we accept that we can't. 
Instead of inner contraction and pain, peace quietly envelops us.