Trying to be honest with myself about what was my part in various relationship difficulties, when I was new to Al-Anon, was supremely troublesome. My mind would automatically leap to defend my choices and behavior, while blaming the other person for theirs - I wouldn't be doing/thinking/feeling/saying this, if they were not the way they were! It started with them, and must be their fault.
Giving up being a victim can be a scary prospect. Victimhood leaves us feeling right, and sometimes righteously indignant. From the position of "right" we can feel protected, safe, and unassailable. We can also feel lonely.
I can detach when I let go of telling myself stories about what this other person is thinking, feeling, or doing. I detach when I let go of "why they said/did this" and also let go of deciding that the other person is acting in response to me.
When I can take myself out of the middle, when I let go of my "I'm the centre of the universe" attitude, then I can accept the other person for who they are, without relating their character traits, or their behavior choices, back once again to myself. Before I can detach with love, I need to understand that this is not a way of "letting them off the hook" because the reality is that I don't have them on the hook in the first place, I'm powerless.
When I can't detach, then I'm going to be jerked around by my own character defects. My own mind is going to drive me crazy, and the other person is happily oblivious. Detaching is a way for me to be able to live in the peace and serenity of my Higher Power. I make the choice to let it all go, and turn my focus onto myself, since I'm the only person over which I can have any control.
My part is to accept myself, accept the other person, accept my life for what it is, and be content in that acceptance. My part is to make a conscious choice not to judge or condemn. To choose love and serenity in the face of an opportunity to do otherwise.