The Present

So, the past and future do not exist. The past already happened and cannot be changed, the future hasn’t happened yet, so why worry about it ? There is only the present right ? (Look up Eckhart Tolle, “The Power of Now”). Another point of view is that all actions and decisions are based on past experience (memories) and nothing else, and some people even believe that events in the future can have a retroactive effect on the present (which immediately becomes the past). So which of these postulates (if any) is true ?