Joking aside, I left my partner of 15 years last year due to musical differences. I'd left before, but was pursuaded back when I was at my lowest ebb.
If you're going to do it, you're going to have to go for the sticking plaster method – a short, sharp and complete separation. As another poster has said, put some distance between the 2 of you so you can both get used to the idea without being tempted to rekindle things or have a sympathy shag.
It's the toughest thing in the world – when your best friend is going through such heart break, and the only confidant she's known over the past few years is the one person she can't talk to. It's very difficult to keep away when all your instincts tell you to offer her your shoulder to cry on.
I moved away and had no contact for 3 months or so. She now hates me and is trying to shaft me in the courts, so perhaps it wasn't a perfect breakup, but if one person is still in love, it's impossible to have a truly amicable split.