To do something halfway is to only put half of your heart into it.  Half-hearted. 
This means that the desire or motive is lacking.  And when you put your whole heart into
something?  Why, then there is nothing
that can hold you back.  At a subtle
level, others recognize half-heartedness. 
People sit up and take notice at whole-heartedness, for when one is
fully aligned and motivated, this is felt by others at all levels.  There is a vibrancy that cannot be
missed. 

If you are doing something with only half of your heart as you say it,
stop and ask yourself why.  Often all
that is needed to kick your actions into high gear is a dose of awareness.  On the other hand, that self-same awareness
may alert you that your efforts may be better focused on something with which
you can more fully engage.  Once again,
awareness is the key.  The greater good
is always served coming from a state of wholeness.