I'm writing with greatest sincerity to apologise for my misbehaviour last time. Sometimes I'm so agitated that I'll act not sensibly, which I know I should not. I'm deeply sorry for hurting your feelings last time. And here, from the bottom of my heart, I beg for your forgiveness. I really hope that you would forgive such a foolish guy as me.
Oh, and lastly, I just want to tell you it's getting cold in Korea now. Please also take good care of yourself while studying and don't catch cold.
Thank you so much for reading this letter. I'm looking forward to hearing from you!