It will get better with time. I had to make the same decision when I was about your age. Just remember that she is better off now and cherish your memories. Very sorry for your loss, RIP.
I had to sign off on pulling support for my grandparents (moms side). Had the final say for grandpa and grandma. Hardest decision I had to make, but my grandfather knew it would be easier for me then my mom to make that decision, so I honored his wish.
Only thing that made it easier was knowing I was honoring my grandfather when I had to make those decisions.