Lucylee, I sometimes do the "what if" thing, or get overwhelmed-feeling from too much I'm trying to do at once. It just adds to stress and anxiety.
I find that endorphins from exercise, really help, too.
It's hard to "live in the moment". DD said to me once, "If you have 1 foot in the past and 1 foot in the future, you pooh on today." She didn't use the "pooh" word, but you get the point. Living in the moment is hard, but when you do it, life can be so much easier-flowing.