The feeling you get when you visit your aunt's grave is only natural. When you are not there, you can think about all the good times you had when she was alive; when you visit her grave it confirms her death everytime.
My advice (and it's by no means professional, but I lost my granddad this year, my grandma when I was 21 and my other granddad when I was just 9) is that if you feel to overwhelmed when you visit your aunt's grave, don't go. I don't mean never ever go. But it sounds as if you need time to come to terms with it. Don't feel forced to go. Go when you are truly ready. I'm 25 and lost my granddad in July. I've not been able to visit his grave since the funeral. There is nothing wrong with that. You have to give yourself time to accept your aunt's death and if going to her grave makes things worse, don't go again just yet.
The other thing I would advise is to talk through your feelings with your mum or grandma - they are going through similar feelings of loss and will understand and be able to help.
kiya