You say, “I want to break up with you but you are my best friend and that’s more important to me than anything else in the world, so when you’re ready to talk about it, let’s talk about if we could continue being in each other’s lives even though I don’t want to touch parts anymore.”
Maybe they’ll say, “Fuck you,” and that’s fine. They have some stuff to think about. But as long as you’re considerate of spaces that will probably need to happen, and as long as you articulate as best as you can that you’d still like to be a person they know and speak to, then you’re having the best conversation you can have.
If they say you can be best friends right off the bat, they’re lying.