Think of two people tied to each other with 6 feet of actual cord or rope. Where one goes, the other has to follow or struggle. If the rope is cut, each person is free to go where he or she pleases.
The dynamic in these situations is similar, just in an emotional/mental sense.
Detaching means releasing your attachment to your spouse (aka, dropping the rope) and making decisions based on what's best for you in living YOUR life. If you're attached, then whatever she does that you don't like, or doesn't do that you want her to do with you/for, will bring you down and you'll obsess, mope, dwell on your sadness and fear and negative emotions.
So detachment means being pro-active for yourself and accepting the reality that the old marriage is over, and that the more you pursue or pressure the WAS, the more you obsess over what she says and does, and the more you "treasure hunt" by reading into or interpreting what she says and does in a way to feed your expectations that she will re-commit to the marriage, the more miserable, unproductive, and weak you get as an individual.
The best way through this is become more strong and independent because that will benefit you regardless of the outcome with your spouse.
So detachment means honoring her request to move on, accepting the consequences of that decision and how it affects you as an individual, and making your own GAL plans for your social and recreational activities and doing them on your own. It means focusing on growing and getting stronger as an individual.
It doesn't mean not being polite or pleasant in necessary interactions. But it does mean limiting those interactions to only those that are necessary, and only engaging with her further if she requests it, and doing so in a way that you manage your expectations.
