Kids learning to deal with grief/loss

My daughter is almost 6 years old. In 2015 she lost her great-grandpa, her grandparents moved 5+hrs away and never visit and we had to put our beloved cat down because of cancer. 
We have done our best to explain that she will see her great grandpa again someday and our cat but she still breaks down crying often missing them . As for her grandparents moving away she doesn't understand why they left and I have no clue how to explain it because it was for their own selfish reasons. Even my husband can't explain his parents behavior. Our daughter has said "grandma and poppop don't love me anymore and that's why they don't wanna see me anymore! Why can't I see them?" Since I don't know how to respond to her statements about her grandparents moving away, I usually just change the subject. My husband told his parents what she has said and feel but they won't call or make an effort to change her opinion. They want us to visit them and make the effort to make contact but no reciprocation of them doing the same in return. 
I want to take away all the hurt feelings and grief at loosing loved ones but can't. It kills me when I see her breaking down crying. Tonight she broke down crying missing our cat and wanting to snuggle with her again. I found a stuff animal that loves just like our late cat which she brings everywhere with her but she wants her real kitty back. 
 She is so young and shouldn't have to shoulder such issues back to back like this. I know loss of loved ones is a fact of life but the issues with her grandparents moving away is not a normal occurance in life and is just compounding issues. 
Any advice on what I should do or say?