I know how you feel. I'm 20 years old, my beliefs in my religion are really strong. I convinced myself to so many things, pondered on so many countless thoughts that I am practically immune to fear. But unfortunately it was never enough to stop worrying about my parents. Now, my parents aren't smokers. They hardly smoke at all. But I still worry about them. I always say to myself "God forbid that something happens to my dad at work! God forbid God forbid!". My mom is always home, so I don't have that much worry for her. But my dad is an electrician. I always think about his work, and what could happen to him at any moment, even though he is outstanding at his job. So, I call him every day, talk to him for a few minutes, just making sure he's OK. I ask him about his work, how he's doing, and tell him how my day was. The worry just fades away then.
But should anything ever happen to my parents, I'm training myself, from time to time, to think, and say: "It was just the time for him/her to leave... God bless you, mom/dad."
My point is, eventually, your parents will die, but you will die someday as well. Just know that, even when they die, they'll be in a better place, and will always be proud of you.
It's just the way it is. No one can change it.
Also, just accept your feelings, cry away sometimes if you must, but don't let it take over. All your emotions are normal. Always keep a bit of strength with you to manage that emotion. Eventually it will fade away. Trust me. It's what I am doing everyday :)