@Paloma153 Yes, that is sad. I have lost things forever by leaving them behind somewhere. Sometimes they get returned, but mostly not.
Don't necessarily be sure the host is being dishonest and just doesn't want to return it, although that is certainly possible. It could have slipped under the bed, or behind a piece of furniture and hasn't been found yet. Or maybe the housecleaner took it.
There's not really much you can do except message the host again, without being blaming, and say you would so much appreciate if they do find it, to please let you know- that it is of great sentimental value to you, and you will cover the cost of mailing it back.
If we attack someone as if they are thieves, when it was our forgetfulness that was the cause, it will make them defensive. If you explain why it means so much to you and say maybe it just slipped behind the furniture and could be found later, even if the host did take it, you at least give her a way to save face and say she just found it, if her conscience leads her to decide to return it.
Travelling with irreplaceable items that you would be devastated to lose really isn't a very good idea. Precious things should be left at home.