The problem is that by saying we need to let go, we are falling into the trap best described in the ancient Chinese poem, “Affirming Faith in Mind,” ... “Seek movement and there’s no movement, seek rest and no rest comes instead.” The more we seek, or better put crave, to stop something or create something, the more just the opposite happens. It is one of the more frustrating attributes of our ego thinking-mind.
The answer to this conundrum is to affirm your understanding that these feelings are all in your mind, that all is one, have compassion for yourself and others ... to turn you will and your life over to the care of your true Buddha nature and be present, free of the intervention of your thinking mind..
For example, let’s say that someone you know said something to you which really pressed your buttons about yourself and upset you very much. So much so that you felt like cutting off the relationship.
On reflection, you understand that this person is a good person and cares for you, but has suffered like the rest of us. And because he is not spiritual, he does not have the capacity to have empathy for you and so lashed out at you in a way that he probably lashes out at himself. He does not have compassion for himself, and thus not for others in similar situations.
You say to yourself that you have to let go of this feeling of ill will that this experience has caused you to feel towards him. But I guarantee that you would not be successful in letting that feeling go. It would hover around you and disturb your peace for days and weeks.
What you need to do instead is to focus on your compassion for him, on his suffering, that he is a product of his experiences just like we all are and has little control over what he does or says. Feel loving kindness towards him and realize that his comments have nothing to do with you, they do not reflect on you, but rather have everything to do with him and his ego thinking-mind.
If you focus your compassionate heart towards him, if you feel at one with him, the ill will you felt towards him will melt away. As the Chinese poem also says, “Just calmly see that all is one, and by themselves false views will go.”