In my current life - there is no one that needs to be let go, and no one that I wish I didn't know. I keep my life pretty close knit, and only invite people into it when I want them to be there, and they want to be there as well.
I don't see the point of wasting time on someone who doesn't want my friendship, love or attention.
That being said - in the past, I have held on to someone for too long. There are parts of me that wish I never knew them, because in the end, they hurt me. They hurt my heart, and those wounds take some time to heal. But, ultimately, they weren't supposed to be part of my life and once I accepted that, it became much easier to let them go.