1. Pick apart everything they do.
Everything you once found charming or endearing is now intolerable. The way this person chews their food, the way they snort when they laugh, it's your least favorite things about them now. Bonus points if you pick and choose when these things bother you.
2. Convince yourself that they only care about their own thoughts and needs.
Assume everything they told you in the beginning was a lie. Anytime they do anything remotely selfish, allow yourself to strengthen that assumption. Bonus points if you work it into an argument that has nothing to do with selfishness.
3. Stop communicating.
Stop vocalizing your thoughts and feelings (they don't matter anyway, right?). Stop vocalizing your appreciation as well. Bonus points if you tell them you guys "don't feel right" or you feel like something is "different" or "off."
4. Stop caring.
Let a couple of texts go unanswered. Hell, silence your phone. Stop checking to see if they texted back. Stop caring if they call you before bed. Use a lot of phrases like, "That's nice" or "Cool." Bonus points if you tell them, "You just don't care anymore." Projection is key.
5. Walk away.
Tell them you can't do this anymore. Bonus points if you challenge them to leave instead. Extra bonus points if they do.
6. Realize you were wrong.
You took a good thing and turned it into something unrecognizable. This person was more than just a checklist of things you wanted. This person was all about "the little things" and appreciated you. This person wasn't just another bandage for your loneliness. This person chose you everyday and you treated them like they were just a stepping stone on your path. Resent yourself because of what you did. Bonus points if you text them at 2 A.M. spilling your guts to them and telling them how stupid you were.
7. Try to win them back.
Realize you both have faults. Realize how much of a coward you were. Realize that this person may not give you a chance. Bonus points if they do.
Extra bonus points if you reach this step before step five.