I am FREAKIN OUT!!! What should i do to make myself feel better?

Ok, so im 14 and work as a volunteer at our towns public library with their summer reading program for children. I love the volunter job and love working with the little cuties that come play the reading program game. But today i forgot about my shift! Idk why! I like totally blanked out and forgot about my 5-6 pm shift!!!! I called and apologized and told them i truly forgot! But inside i feel like fricken crap!!!!!!! HOW COULD I FORGET!!! I feel like such a loser. I could i possibly do to calm myself down????? Right know im like crying!!!! Omg!!!!Help!!! Plz no rude comments! Thank you!!!!!