Most Helpful Guy
@emerald77 gave you some very good advice I strongly agree with. Especially if the person is irrational and manipulative, sometimes your only choice is to get away first and then tell them in whatever form you're comfortable with that avoids all the drama they'll create in trying to convince you to stay with them.
The fact you're aware the relationship is now unhealthy is a good indicator the initial rush of excitement and emotional bliss has been replaced with the reality of who that person really is and what you're are actually dealing with day to day. Sure, there's still some attraction and some aspects that are good, but when the bad starts outweighing the good, and either the other person has no desire to work on changing things, or you guys have tried and the outcome is still the same, it's time to close the book.
I'm impressed you used words that didn't completely implicate the other person, but rather you're acknowledging the relationship just isn't working out for either one of you. This will help you process things much faster and you seem to be very rational about it.
If you think he/she can handle the truth, then perhaps you can tell them in person. However, if you suspect he/she will act out like a child and become belligerent, before they start begging to give them another chance, then perhaps you should just get your things in order and they will figure it out.
Good luck and trust your instincts.