‘Forgive us our trespasses as we forgive those who trespass against us.’  Jesus is not saying that God’s forgiveness is conditional upon our forgiving others.  In the first place, forgiveness that is conditional isn’t forgiveness at all, just fair warning; and in the second place, our unforgiveness is among those things about us that we need to have God forgive us most.  What Jesus is apparently saying is that the pride that keeps us from forgiving is the same pride that keeps us from accepting forgiveness, and will God please help us do something about it.  ~Frederick Buechner, Beyond Words, page 119   

What would we need forgiveness for if we never messed up when it comes to forgiveness?  Surely God gives us no free sin passes like one of those coffee cards you get where they punch it every time you visit your favorite cafe and buy a drink of some sort (buy nine and get the tenth free).  Behave nine days and on the tenth day you can sin as much as you like isn’t what God has in mind for his children.  But we have to figure he’s prepared a way for us when we do sin, don’t we?  What are we to do when we haven’t been forgiving and we find ourselves needing forgiveness for that?

If there is no possibility of us being unforgiving, then there is no possibility of us falling short of God’s own glory and perfection.  But we do fall short (Romans 3:23).  You see, only God forgives perfectly.  We forgive imperfectly just as we love imperfectly.  We wrestle with our weaker selves and often take days, weeks, months, and sometimes longer—to actually forgive.  

8 Do not remember against us our former iniquities;
   let your compassion come speedily to meet us,
   for we are brought very low.
9 Help us, O God of our salvation,
   for the glory of your name;
   deliver us, and atone for our sins,
   for your name’s sake!  ~Psalm 79:8-9, ESV

As Buechner says, our unforgiveness is among those things about us that we need to have God forgive us most.