Do you believe in the power of positive thinking?

I've had an awful lot of bad luck and awful things happen to me recently and I tend to think very negatively. If I think negatively then anything good that happens is a bonus! (Not that it ever does...)

There is an awful lot of positive type posts on facebook that I follow and cringe at whenever I come across them.

So, what are your thoughts on this? Does positive thinking lead to positive things happening or is it a load of bull?