write a heartfelt apology for having lost the priced possession of a dear friend.
Dear [Friend's Name],

I am writing this letter to apologize from the bottom of my heart for losing your prized possession. I know how much that item meant to you, and I am devastated that I have caused you pain and disappointment by losing it. I understand that no amount of apologies can bring it back, but I want you to know that I am truly sorry for my carelessness and the hurt that I have caused.

I am aware that this item had sentimental value for you, and losing it has caused you a great deal of distress. I cannot imagine the pain you must be feeling, and I am truly sorry for contributing to that pain. I want to assure you that I am doing everything in my power to find the item and bring it back to you. I know it may not replace the item, but I hope it can bring some comfort to you.

I take full responsibility for my actions and the consequences that have resulted. I know that I have let you down, and I am truly sorry for that. I hope that you can find it in your heart to forgive me for my mistake. I promise to be more careful and responsible in the future, and to make it up to you in any way that I can.

Once again, I am so sorry for losing your prized possession. I value our friendship, and I hope that we can move forward from this and continue to support and care for each other.

Sincerely,
[Your Name]
Thank you, can you re-write this to be come across as overly sympathetic and sarcastic?
My dearest [Friend's Name],

Woefully and with a heavy heart I prostrate myself before you with an emotionally flagellating letter to denounce my part in the tragic chain of events that led to the loss of your most prized possession.

The world itself mourns at the loss of your item. Know that the rest of humanity is equally devastated as I... and that none can hope to fathom the grief felt by you. 

I've let you down in the worst kind of way. Even now as I write, the guilt is immeasurable. I understand that no amount of apologies can undo what has been done. Words evade my hand as I fear there are none to truly describe how sorry I am. 

Without a doubt your treasure is completely irreplaceable and the memories you've had with it are now broken, shattered and fading.

Would that I could hope to take responsibility for this atrocity born  of my own complacency... alas I am afraid that would be futile. A futile burden that I now bear. Know that I will dedicate what's left of my pitiable body and mind to find and renew that which was so carelessly lost. 

I would not dare ask for forgiveness, only for atonement. Will you grant me the chance to make things right once more?

Sincerely,
[Your Name]