Letting go

There comes a time in our lives in which we must let go...of people, of things that we hold dear and true but they are unhealthy for us.  They bring us sorrow and pain.  They perhaps do not mean to but they do and we know that the only way we can truly realize who we are or our potential is to move on from them...and yet we do not.
We hold a belief that they will change, that we can help them, but ultimately only they can change and if they do not want to there is nothing we can do.  So even knowing this we still stay.  Why?  What purpose does it serve?  Is there a lesson to be learned in this design?  Is the design for us to help THEM?  To give them strength and courage?  To see more?  Is there a design for us as well?  To perhaps be certain of what we want and need and to give us courage to ask for it?
I am not an expert in these matters for I struggle nearly every day with them.  I seek only to understand the momentum of the human soul and my purpose in it.  I am unfulfilled and unsatisfied that this is all there is and all that I deserve...and yet I do not know how to ask for what will fulfill or satisfy me, lead alone what I deserve.