There's no real formula to it. Just know that your mind cannot conceive of a negative concept. "I want to forget her" will just make you think of her. So when people tell you to busy yourself, they are not wrong.

However, it can also mean you were just running from the feelings. When the thought resurfaces, all those feelings return and you're back to square one. You need to spend some time to actually process your feelings, and come to terms with how things are now.