The sentence adverb regrettably is good for expressing regret, or sorrow, about an unfortunate event. If you forgot that today, May 5th, is your brother's birthday, you might say, "Regrettably, I thought today was the fourth."
Regrettably is often used at the beginning of a sentence that tells something you wish you had done — or hadn't. You can also use it in the middle of a thought to signify sadness, apology, or regret, like when you're lost in the woods with a guide who's regrettably choosing one wrong path after another. Regrettably also describes things that are disappointing, like your regrettably brief career as a fire juggler.