My Prius fuel gauge is off, and it's freaking me out!

My fuel gauge has been acting strangely lately, and it's weirding me out! I started this week with a full tank of gas, and after about 70 miles of driving on the odometer, the fuel gauge dropped to just under half a tank of gas. (I usually get 230-300 miles per tank, so this can't possibly be right.)

The same thing happened just the other week. I had only driven about 100 miles when the fuel gauge went from mostly full to barley full. I freaked out and went to the gas station. Only, my car would only accept 2 gallons of gas.

My question is: can I reset this so that it works properly? If it can be fixed, is there a simple way I can fix it, or do I have to take it to the dealer?

If anyone else has this problem, is it even worth fussing over, or can I just leave it as is and fuel my car when my odometer says I've gone a certain number of miles. For example, right now I know I have at least another 200 miles left for this tank of gas, but I don't want the fuel gauge to drop too low and freak me out while I'm driving...