It's very sad that Monty turned out to be unwell, and having to choose to put a pet to sleep is a very hard choice. But it was better than letting her live in pain for who knows how short or long. You loved her very much and gave her 21 wonderful months of life (many hamster-years!), and she knew it.
It is said that treasure can be found at the end of a rainbow, and Monty was, no doubt, your wonderful little treasure.
~ Forever loved, enjoy your treats, and playing up by the rainbow bridge Monty.