Date: __/__/____ (Date)
Dear _____ (Name of the Girlfriend),
Through this letter, I would like to apologize for the misdeed I have done on __/__/____ (date) due to the reason ______ (mention reason). I know that I should not have got indulged in such a behavior.
I was going through some ___________ (mention issue) due to which I was in high stress which came out in anger and I __________ (scolded/shouted/other) at you, due to which everything went worst. I should have not done this to you. I request you to kindly accept my sincere apology and forgive me for the same. I ensure that such behavior would never be repeated and also, would not treat you in such a way.
I believe you would consider this as a genuine issue and accept my sincere apology.