Since you asked - I was a coward, said I was visiting my brother for a few days, packed a suitcase much too big for a few days, and did it over the phone once I was safely across a state line. I don't particularly recommend that approach, nor am I proud of it, but it did work.

I tried not to see them face to face, since I was sort of lacking in spine at the time and figured they couldn't talk me into coming back if I didn't let them talk to me. When we did talk, I avoided the language of blame entirely (although I tend to do that anyway) with things like "we never went anywhere good together", "we're stuck in a rut and neither of us is happy", "we're poison for each other" etc. Lots of "we", not much "you" or "I".

I felt awful for a good long while about it. But I took comfort in knowing that each of us would be supported by our families and friends, so neither of us was truly alone. We really were poison for each other.

I don't know if that helps at all since it might be nothing like your situation. But I'm happy to talk if you'd like.