Often times, many people will say to someone in your situation, "There is nothing you can do, so stop worrying." While these folks may have the best intentions trying to impart some sort of knowledge they assume you don't know of yet, it seems pretty obvious, right? You cannot do anything about ALS or any disease that strikes a family member - but it still causes worry and unneeded stress which can wreak havoc not only on your mind but on your physical health as well.
I'm not a HUGE proponent of self-help books because many of them are pretty obvious. Sometimes self-help books are so obvious, you can actually feel the money disappearing from your pocket for no reason at all. I'm going to recommend 'The Power of Now' to you in the hopes that this book will do for you what it did for me.
Forget the tired phrases 'one day at a time' - and really live second-to-second. What's happening around you right now that you can control? Is everything fine? Then relax. I recommend learning meditation techniques so that your mind is not running wild with 'what ifs' and 'this might happen'.
Stress is inevitable, but you can act as gatekeeper over what you can control and what you cannot. The only cliche I'd like to offer is that it's always darkest before the dawn. I believe you'll find something helpful for you to exercise these feelings and manage them in a healthy way.
Link to The Power of Now:
The Power of Now