I baked carob brownies for tu b'shvat with butter. The next day, I made pareve barley soup and a dairy dessert. I took these (& other pareve stuff) to my son's house for a tu b'shvat seder. They asked if the soup was pareve, because they had eaten meat. I forgot about the brownies, & I said that everything was pareve except the dessert. The next day, I realized that the brownies, which everyone ate, were dairy. So, I had put a michshol lifnei iver due to forgetfulness. I feel terrible. What should I do to atone?
There's good news, and there's better news. The good news is this has happened to many others before you, and the better news is not to worry about it. (Well, try to educate yourself and be more careful, but it needs no atonement.)
As Fred noted in his comments, as long as the dairy was a different meal than the meat, you've satisfied the bare-minimum rabbinic requirement, so we're just talking at the level of custom (according to many).
But I'll tell you a better story! Years ago at Ner Israel yeshiva near Baltimore, there was a complicated mix-up and they served real, dairy ice cream instead of pareve ices for dessert right after the shabbos meat meal. The kitchen coordinator felt terrible, but the mashgiach told him not to worry about it. The Nesivos HaMishpat writes that a mistaken violation of a rabbinic prohibition needs no atonment. (And as long as the ice cream and cholent weren't cooked together, it would be rabbinically prohibited; and as above, if a different course, it might not even be that.)