There are many self styled Gurus and advisers on how to go about it (though I wonder if they themselves can!) and are full of ideas and methods to "guide" ....but most often it is a sort of misguidance.
WHY is it so?
Because every action we take as per their "guidance" reinforces the fact that there exists a problem (in this case - not being able to let go of past).
HOW to overcome this? (click)
You cannot ! .... till such time understand why we canoot.
Humans are programmed to remember. As are all living beings, but we are also blessed with a higher mind, a better brain and hence larger capacity to store - to last us our life time.
We have to understand that we ARE SUPPOSED to have memory and its effect . It is a good thing too and meant to take us through are lives unharmed, creating perspectives and frames of references. So we cannot wish memories away.
Also, remember that the happy events remain in background while unhappy or not so happy ones surface more often. That CANNOT be avoided. That is the way we are programmed. Else we would not last i this world. THIS is what saves us from dangerous situations like falling, burning our hands, downing, etc, etc.
The trick is to -
1. Understand the Triggers- what brings the past memories, good or bad, to surface. Thereby avoid it. Eg, if going to a museum or a particular lakeside reminds you of someone who was close but you want to forget; then its better to avoid going to that place.
2. Remain Rooted to the Present - Past is just a frame of reference. People come and go in life. One should not get stuck on past events. Remember, If you loved someone in the past does not mean you must cling on even after that someone has gone or has decided to move on.
So should you.