Letting Go

Today I was asked to let go and not harbor bad feelings towards another person.  Sounds easy right?  Not for me.  I don't know what it is but I have a difficult time letting go of things that make me upset.  While I was mowing this afternoon I realized that I don't know how to let things go.  I literally don't know how to just say ok and move on.  I have always just stayed pissed until so much time has gone by that I don't even remember why I was mad or I realize that it was over something really stupid but I still can't bring myself to let it go completely. 
I don't know any different.  While I love my father to death he is a very angry person who to this day still goes on rampages about his ex wife. (They have been divorced for over 19 years)  I know I have picked this habit of his up and I don't want to harbor things like this.  I have actually cut people out of my life because I can't let go.  A few of my best friends gone, my "mom" gone, others who I have cut out but they are back in.  I just don't know why I can't function mentally and just let it go.  I can feel the anger in my body and I don't like it there. 
Over this last year as I have found the Lord it has really been weighing on my heart.  I know that this anger I hold on to is not what the Lord wants me to do.  I feel that I can't be a true servant of the Lord until I get over this bridge.  In my eyes this bridge is old and falling apart I feel that even taking one step will bring the entire structure down.  Where do I begin? How do I begin? May the Lord help me and I know I will get more opportunities to work on this in the next eight months.
I hope you enjoyed my thoughts.