In reality our existence is composed of endless consecutive Now moments, one after the other, and another, and another. We always exist in the Now, although our minds may be all over the place, spending a lot of time in the past and in the future. But the past does not exist, nor does the future. The past is just an accumulation of memories that live in our mind, while the future is something we are creating right now in the Now, but that has not happened yet. The past was Now at one time, but now that it's gone it's only a memory and no longer real.
In our minds we keep thinking about experiences from the past and projecting them into the future, creating fears and worries. We anticipate and expect that unpleasant experiences from the past will be repeating themselves in the future (and the more we do, the more they may). So then we worry about the future, and strategize and plan to avoid that projection from happening. Hence, we live in our minds, which live in the past and in the future - which both don't exist. Crazy, right?
If we simply lived entirely in the moment, in the Now, and left that non-existant past, and that not-yet-created future alone, there would be no need for fears and worries. How great is that? Think about it.