I was apparently very depressed when I was in Primary 3, I think thats about aged 8? My best friend and her family had to move down to south wales, we lived in scotland
my friend and I had been inseperable since we were 3 and had to wear same clothes/ eat meals together etc etc they only lived a few doors down and when my dad got up to go to golf at weekend my mum banned him from opening curtains as she would see and come round, it was only about 7.30am!!
am only telling you this long yarn because when they left my world fell apart and I was too young to understand why - my mum said it was like a black cloud had come over her lovely sunny child and nothing they could do would shift it
they bought a dog and that gave me something else to focus on and care for
not am not saying you have to buy a dog but I do not remember any of this. I can't remember being sad or depressed, I only remember the day our dog arrived. Children are very susceptible to moods/worries about them and have a need to feel similar emotions to adults, they maybe just don't understand them.
she will get through this and you'll be left wondering what you were worried about. It sounds like you have a lovely dd and you have a great relationship with her - you're doing the right thing by not belittling her feelings or ignoring her questions. please be more confident in your abilities and she'll have moved onto something new soon
hth if you've made it to the end and not too bored
btw - am still in touch with my friend in wales and we had our kids at the same age and went to each others wedding - I do like a happy ending