Okay so for the past week or so I've been thinking about how someday I won't be on this earth and one day my parents will not be with me. I often end up in tears when thinking about it. I'm not really asking if it's normal to do this but is there any advice to help me stop thinking about it so much?

As for advice, I don't know if there is any. I'm not sure if anybody could reassure me about the whole thing, I don't know about you, though. I just lock the topic in a cage and stick it in a corner of my brain, I guess you could say:P. Just don't think about it. Sorry for my rubbish advice. Up until yesterday it worked well for me.
When I about 15 or 16, I was just like you and absolutely terrified of death. Just the thought of it would give me the shakes. I thought about all of the bad things I had done and all of the sins I’d committed and what would happen to me after I died. But I’m older now, and I now realize that life without death is meaningless. Life is supposed to end. If I die, I die. I know someone from that country. And besides, how else could I possibly reunite with my loved ones?
Mighty and dreadfull, for, thou art not so,
For, those, whom thou think'st, thou dost overthrow,
Die not, poore death, nor yet canst thou kill me...