Mindfulness is about focusing our attention on the Present, the Now. So much of nature, everyday life, person-to-person interactions, and present things you should be thankful for when you focus your mind and time, thinking about the past and/or the future. Often, it brings forth the energy that went/goes with those times.
When you focus on the past you either romanticize it or sometimes, overdramatize it. What I mean is. Many times you remember certain things and just how wonderful it was. When the truth is many bad things were happening at the same time. Likewise, when you remember some awful situation you went through, you remember only the bad of the moment, not that some things that were also wonderful and good happened. As someone once said, "The good ole days weren't always good." And I say the bad ole days weren't always bad, and I am aware that there are exceptions to this.
There's nothing wrong with remembering the past or thinking of the future, just that constantly thinking about the way something used to be can end up making you extremely unhappy right now. Also, thinking about the future could cause you to miss out on things happening around you right now and make you unhappy about what you don't have.
Remember, you can't live in the future. All you can do is prepare for when you get there, and since you don't know the details of the future. All you can do is the things you are currently doing to the best of your ability, for tomorrow will take care of itself.
Don't forget. You can neither change the past nor control the future. The only thing you can do is work here, in the Now. So, please be present in your own life, your mind, your body, your world, and make the best decisions you can, for the present, you. That, however, does not mean ignoring the past or the future. You should absolutely dream and aspire, but not be obsessed. Never, never obsess. Instead, use the past as inspirations and the future as aspirations, but deal only with the Now. Be Mindful of how you're dealing with others, now. How you're treating your environment, now. How you're taking care of your finances, now and how you're taking care of yourself, now. You will find that in the end, everything else falls into place.