I wrote it before we talked so I could get my head on straight. It has every little thing he's ever done wrong and it's written very aggressively. It's a "shape up or ship out" letter. I had it tucked into a magazine and forgot about it because we had that talk and he truly has been improving. He came downstairs and said, "wow that's a wildly hateful letter" and left. I knew right away what he meant. I thought is thrown it away- magazine and all. Sh!t. What do I do now?!

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Re: Oh fvck. DH found an angry letter I never gave him.
Just tell him the truth, it was a venting letter that you had never intended for him to read.
this. Tell him to think of it like a diary entry - you were writing to get a hold of your feelings. I'm assuming you were upset/angry when you wrote it. And if it was meant for him to read, tell him that you considered unloading it on him but decided against it.
No joke, this just happened with my DH and I. I packed our bags (since the movers were packing things) and I guess I packed this journal thingy that I write letters to him when I'm super pissed, in his bag. He found it while I was gone on vacation with my parents and Annabelle in Atlanta. He called one day and was furious, but I explained to him that if we both said everything we felt and wanted while we were angry, we'd really hurt each other. I said that it was easier for me to write down my feelings instead of holding them in and that he was never meant to read those things. Thankfully he had a few days to cool down before I came home, but he's been a lot less irritating since. lol
I also vote to just be honest. You didnt mean to hurt him, and he should realize that. GL!
Yeah, he was hurt, but I mean who wouldnt be; you know? Its just one of those things that both sides has to just kind of realize that no one was meant to be hurt in this situation and its not your fault or his. Its just a sucky thing that he found it, but I'm sure he's thought equally bad things about you and just not voiced them (or wrote them, lol). It doesnt mean you're wrong for having written down everything, and it doesnt make him wrong for having read it.
I actually found that him reading it (even though it was hurtful) was helpful. He now knows how I see things and oddly enough, after we talked about everything he read, our communication is much better. Its just one of those things that you both have to step back and try to see it from the others POV. You didnt mean to hurt him and he should be able to see that, while you just have to realize that it was probably really hurtful to see that you felt that way. As long as you both communicate, it'll pass and you'll be closer. :]
Maybe this will be the conversation where you both can just be like, calm and clearly state that while the format was wrong, knowing how the other feels is beneficial. KWIM? The only advice I can offer is just sit down and just be receptive that he may be defensive and he may be really hurt, and that's alright. The whole thing is just keeping the conversation calm, you know? At least he knows the deal now, even if it did hurt a little, its out there and now all you can do is work on it. I hope it works out!! I'm sure it will!