Here is the letter with prepositions added to fill the blanks:
Dear Paula,
I’m writing to apologize for shouting at you last weekend. I’m terribly sorry for the argument we had, and I have been feeling so awful that I had to write to you.
I know I upset you, but I didn’t mean to. It was stupid of me to get so angry with a silly shirt. You know that I don’t mind people borrowing my things. Recently though, when I have lent different things to friends, they have given them back damaged. Instead of saying anything to them, I got angry with you. I wish I had kept my mouth shut!
You’re my