It sounds as if you are just a very intelligent, thinking and extremely creative person.
I have the exact same worry/overly active imagination tendencies and have a 13 year old DS with the same problem. So, I'll tell you what works for me (and him). I funnel my creativity into art. Some of it's a bit strange, but it channels my creativity in a positive way. Think of it as an imagination overload valve. Reading is also a wonderful activity. But, have you considered writing? It can be stream of consciousness type of writing too, if you just need to off load. My son makes amazing puppets, ala Hensen style. It keeps him focused, creative and helps to distract him from fretting. Getting involved in theater or music are both great options. This is where the arts are your new best friend, and you have the kind of mind that will excel in them.
Many people say that they are jealous of those with creative minds, and in truth I wouldn't want to be any other way, but it does come with a cost. Fretting is that cost. But, finding a way to channel your amazing imagination will do wonders for you.