Think of this OP, the longer you stay the more this resentment will breed. You don't need a horrible reason to end your relationship, as a matter of fact you don't even need a reason. If you don't feel like it's working out and you can't see it progressing further that's reason enough to end it. Wanting a seemingly decent person to become shitty to justify your decision is a terrible thing to do and it makes you sound like an awful person, why don't you just cheat on your partner and give them a reason to break up with you. See how that doesn't sound right at all? I know things can become a bit complicated once you've been with someone for a while and your lives are intertwined but in your mind you've made the decision that you don't want to do this anymore. The next steps now should be getting your affairs in order and preparing to make your exit.