How to emotionally detach from someone?

The only way I know to do that, is determine to, once and for all, and practice it.

Think of all their bad qualities, constantly.

Resentment always works for me

open up to them and let them show you how to shut down emotionally.

This may sound a bit hard core for you but kicking them to the curb and forgetting they were ever a part of your life

talk to them and get to really know them

Get drunk and find another lover

Attain closure.