It might help to ask yourself what it is that you are worried about the future? Are there any steps you could do to prevent such an outcome? Personally, I found that writing down my thoughts helps a lot. Not only does it help to focus on the particular problem, but in the long run I see patterns of my fears and anxieties.
Regarding reading on the subject: I really like the book "How to stay sane" by Philippa Perry (The School of Life). It is a small, sober self-help book and as far away from the myriads of questionable pep-talk guidebooks as it can get. It also touches self-awareness which IMHO is key to sane "living in the moment".
Maybe take a course on stochastic processes to clear your mind. (I'm joking, for the record. But hopefully the point has been made.)
So when worry arises, you might try labeling it in your mind: "Worry, worry, worry." That may sound trivial, but you may also find it calming.
Can you really be defined by a decision you once made?