I know I am a bit late to this thread but I just wanted to say my heart absolutely goes out to you....and yes it has happened to me, and of course I was absolutely devastated.
A little bit of backstory: my whole life I was told off for being careless and losing things, amongst many other negatively perceived personality traits. I had a neurodiversity diagnosis as an adult and that helped me to make sense of it.
However, a few years prior to that diagnosis I received a very valuable piece of jewellery from a beloved deceased family member and irretrievably lost it.
I think for me what worked was being really kind to myself, reminding myself how many challenges I have overcome due to what is essentially a developmental disability. Of course you may not have any kind of neurodiverse diagnosis but the thing that you are going to tell yourself kindly doesn't necessarily have to be in connection with the action of losing the item, even though it was for me. I'm sure there are any number of "brilliantnesses" in your life, personality and character that you can remind yourself about!!
Now in situations like ours, sometimes the hurt can be connected to feeling that you have failed the person who originally gave you the gift. And if you don't mind, I'd like to tell you something that I found helpful here:
I once read a description of heaven as being a place where our personalities were essentially who we are, but all the obstacles, hindrances and challenges that make up our day-to-day lives are taken away from us, and thus we can see clearly. Now, I am not a Christian and do not believe in heaven, but I have adapted that to imagine that if my deceased relative was in communication with me over the loss of this gift, they would have had to pass through one of the biggest changes in life, which is death, and because of going through that big change, it could be said that the obstacles had been taken away from their eyes and they could see clearly and be the best version of themselves.
So in this context, whilst my relative may have bemoaned my carelessness when they were alive, I ultimately believe that they are such a loving person that if the obstacles of life were taken away from them and they could see clearly, they would be throwing me so much love, compassion and forgiveness and desperately wanting me to take that on board. Because the 'bemoaning my carelessness' part would have been because of a myriad of reasons that are connected to the obstacles that life places in front of us: the belief that certain things have a value when ultimately they don't, concerns about my ability to survive in life if I am careless, their own guilt because they gave it to me and not to a different family member, or even the unrelated fact that they found out on a Monday morning when their tax return was due!!
Whereas if the obstacles were taken away and they could see clearly, they would be able to explain to me that rocks from the ground are really no more or less valuable than a flower, my adorable kiddo's curls, or my friend's palpable, chatty joy in her new career-change course.
If the obstacles were taken away and they could see clearly, they would be able to explain to me that the love that inspired the gift was absolutely the only important thing about it and that that love is immutable, ceaseless in its impact on my life and personality, and will never ever, ever be lost.
P.s. It helped me to literally imagine them sitting on the end of my bed and hugging me, and to try to actually imagine the sound of their voice, their intonation and the words that they would use. As an added bonus: In a "Hilary Mantel" sense (her memoir "Giving Up the Ghost"), once you've imagined into being your lovely grandma on the end of your bed, manifesting the most loving version of herself, she may stick around and come to personify the kind voice that you direct at yourself in future.