sadness wrapped in happy wrappings

Sadness. Somehow even though everything seems perfect I still seem to find depression. I know that's what bi-polar is all about, the ups and downs and blah blah blah, but what if there are no ups. Just downs. Then what. What do you do when even though something amazing is happening in life you can't help but stress about it. I feel like even though everything seems fine something bad is gonna happen. How am I supposed to be happy about anything when all I can do is stress about the what-ifs....