I am sure people here will have a variety of feel good advice for you. Realistically though, this is a very personal conditioned response to some stimuli (in this case, presumably thoughts of your mortality).

The Buddha had a similar experience during his time as a ascetic living alone in a jungle that was more dangerous then than it is now.

http://www.accesstoinsight.org/tipitaka/mn/mn.004.than.html

The short of it is that, while experiencing panic or high anxiety, he would stop where he was and take notice of what was happening. He would say to himself "this is fear and terror". He would stay like this until it subsided.

When you say that you try to stay " in the moment " are you trying to do so as a distraction from your panic? If so, this won't work. Because in that moment you are experiencing panic. You can't stay in the moment without allowing yourself to experience what is going on in the moment. This is a frightening prospect to most people and does indeed take great courage to do. I draw an analogy to the first time I ever went rappelling. To go rappelling, you need to put all of your trust in the rope and other equipment and let your body slide back to be parallel with the ground far below. This is terrifying at first. But after you experience it for a while, the fear subsides because you come to know that that you will be fine. This is not an intellectual knowledge. You can watch people do it and intellectually know you will be safe. But it is a different indescribable knowledge when you experience it for yourself.

I think panic and anxiety is like this. Panic especially isba purely physiological phenomena. Adrenaline is pushed into your body, your heart rate, blood pressure, and respirations all increase, triggering a fight or flight response. You think something must be wrong because usually this is triggered by real danger. There is no obvious immediate danger, so the mind pulls out the next best thing that you find averse. Now you associate this panic feeling with these thoughts and from that point in, these thoughts alone can trigger the panic. It is a vicious feedback loop. The solution from the Buddha's perspective is to sever this association. You do this by consciously recognizing the physical sensations arising as a result of thoughts. This takes time and disciplined repetition.