Well reality as we know it tells us that we are not really capable of living in the present because it takes time for the information about the present to be processed by our brains so as to comprehend it. In that small infinitesimal piece of time the present is lost becoming the past. We are always living in the past and everything is just nostalgia for the now, we are all history living in the past.
As for the future being behind you it could never be because it has not even happened, it does not exist only as an abstract idea inside the head based on past information.
Living like there's no tomorrow when there isn't one anyway.
But if this question is a metaphor then it might mean, stop dreaming about what might or might not be and just get on with your life. Put it all behind you and just get on with the present.
If you have a terminal illness then worrying about the inevitable will not help your situation between now and the inevitable so the best thing to do is put the future behind you to enjoy the now.
On the face of it putting your future behind you seems like a good idea except that all successful people make plans and a plan is only a map of actions to perform in order to get to a future result or a guide to get you somewhere in the future or how to get there if you want to or carry it out.
As it stands the future or the present doesn't exist in theory but in practice people make and carry out future plans everyday successfully. This means that metaphorically it could mean put your plans behind you for the moment, meaning that your future is potentially behind you.