Wouldn't it be simpler to say something like I'm afraid I lost the book that I borrowed from you when I moved...?
I lost a book that is borrowed from my friend and I wrote a letter to apologize for it. Could you help me make the sentence natural? Thank you!
I am so sorry that I could not apologize to you in person. About the book that I borrowed from you two months ago, I lost it when I moved to another apartment a week ago because of my carelessness.
Wouldn't it be simpler to say something like I'm afraid I lost the book that I borrowed from you when I moved...?
And "But I will be happy to replace it, or to get you another book you want from the bookstore"?
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.