I came across another similar exercise to do before going to sleep every night:

Imagine you have a box with a very tight lid. It has a special slot you can slip stuff in but you cannot open the box. Now slit into that slot all the things that have happened today that have upset and irritated you left you feeling powerless and angry and frustrated cheated hurt. When you have made sure that you have put them all safely in the box put it by your bed and go to sleep. You will notice that every time you pick the box up it is always light – no matter what you put in it it never gets heavier and whatever you succeed in completely putting into the box doesn’t come out again.