today, people live well into their 80's and even 90's.. you'll still have her for quite some time. my dad was already 60 when i turned 15.. im now 26 and he's 71. (the thought about losing him now doesn't scare me as much as it did when i was 15).. the only thing you could do to help her live to her fullest is DOn't create extra stress for her.
you will be somewhere in your 40's (maybe even your early 50's) by the time she does pass, and by than you'll have your life put together, married, and even have kids of your own so it won't be as scary when the time does come. take a few deep breaths, relax, and enjoy her, make her life as easy as possible and stop worrying about something thats not gonna happen for another 30 years.