I even wrote a letter I was going to give him, breaking it off.
I am not clear if you gave him the break up letter or not. I hope you did. Make it the "official" break up.
You both feel like shit because that is part of the breaking up process.
The stages of acceptance / stages of grief thing.
In time you will both feel better. Keep in mind you are further ahead than him because in being the breaker-upper, you have been thinking it over in your head for a while now. You have had the heads up.
Him as the one being broken up with? He's got to catch up and the first stage there is "shock/denial" like "I can't believe this is happening! I can't believe this is really over! I don't want this to be over! I don't want to see/accept it as over!" and you can expect him to hit you with "bargaining" type talk as he jiggles across the stages into final acceptance.
Best way to navigate that is be kind but firm.
"Yes. It is over. I am sorry. We are friends now, not romantic partners and lovers. "
Being clear with the boundaries helps YOU too.
Hang in there.