It can be so hard to find the right words in a tough situation, so why not say it with balloons! The shock value of this kind of apology on a balloon might just be enough to soften the blow and give you a head start on forgiveness part of the whole thing.
Each package contains 12 standard latex balloons (4 of each color) printed on both sides. You can have them filled with helium, hang them up yourself with a bit of tape, or fill a room if you've really fucked up. Hopefully these help, whatever the situation.