I took my daughter to the doctor this week for her 6-year-old checkup. The night before, and a few times in the morning, we had this conversation:
Me: I don't know, sweetie, we'll just have to wait and see.
Ivy: It will hurt if I get a shot!!
Me: I know it will hurt, but it only hurts for a second and then it's over. Plus, we don't know if you need a shot so let's not worry about it just yet.
Ivy: But what if I have to get a shot??!!
Another example. Someone I know has 2 little girls and told me that his wife watches MTV's "Teen Mom" and worries, "What if the girls end up like this??!!" I think we can all agree that show is extremely disturbing for many reasons (mostly the outfits and the butchering of the English language), and no one wants their daughter to become pregnant as a teenager, but... sitting up at night worrying about your 3-year-old losing her virginity (and her fashion sense) as a teenager is not the best use of mental energy.
We all worry. We all lose sleep over things we can and cannot control. Ultimately, however, we figure out a way to deal with whatever happens, good or bad. My advice to Ivy is to try to make smart choices and try not to put yourself in bad situations, but whatever happens, we'll deal with it, and we'll deal with it together (regardless of whose fault it is!).
If you must spend your time thinking "What if?" - spend it dreaming about fun and exciting things to come, instead of worrying.