Adding to the chorus- a note is a really good way to go. I wouldn't write a long involved letter, just a short I'm sorry and Thanks for your help would be enough for him.

Now, as for yourself, it's going to be a long process to forgive yourself. I posted this to somebody else, but I use this phrase (which I broke down, so I could expound on it) to jumpstart the healing process for myself.
-"This horrible, no good, very bad thing happened." That fact will not change. It happened and it ain't gonna undo itself.
-"It was wrong." No ifs, ands, or buts. It was wrong.
-"I forgive myself. I forgive [insert guilty party here]"
-Now, "I turn it over to the universe"/ deity of choice

I use that as a kind of mantra or make myself write it repeatedly, until that thought starts to come naturally whenever the pain or embarrassment bubbles up- because it will never totally go away- and it makes me feel a lot lighter. Not a cure all, but maybe it'll help you get started.