"Letting Go = Non-attachment, not holding onto thoughts, feelings, experiences; however, letting go does not mean suppressing."
Cardaciotto L. Assessing Mindfulness: The Development of a Bi-Dimensional Measure of Awareness and Acceptance. PhD Thesis, Drexel University, 2005.dspace.library.drexel.edu/retrieve/4264/Cardaciotto_LeeAnn.pdf
Letting go is also challenging because we have a strong tendency to cling to (or be averse to) a set of thoughts, feelings, experiences - these are "sticky" - hard to let go of. Letting go is the more active side of acceptance. When we clearly see how every time we indulge a miserable memory from the past (wallowing) feels awful, we realize that we can choose to let go of the replay button after the first couple of seconds, before the story picks up emotional momentum and carries us away. The same holds for other compulsive, destructive habits - we can learn to let them go.
Non-attachment, which refers to freedom from addictions / compulsions, is markedly different from detachment, which implies lack of caring.