The task is easy or difficult depending on how you choose to look at it. If you make a dedicated effort at, you will find it very easy. On the other hand, if you choose to keep thinking about him/her and have kittens every time you see something which reminds you of your relationship, then it becomes infinitely hard. The first step, hence, is to be sure for yourself that you want to forget your old flame. As the older and wiser lot will tell you, well begun is half done.