It seems like you have generalized anxiety disorder with superimposed depression.
The first step towards recovery is to accept that you have a medical problem and also to realize that there are ways to get over it. Generalized anxiety disorder is fairly common problem, with various degrees in various people. It may manifest as a Panic attack like the one you are experiencing, with symptoms like palpitations and impending doom
Here are my recommendations-
1) It's important that you learn certain behavior modification technique .Although they will take time to master; it's the best way to get over anxiety
a) Continuous muscle relaxation- As the muscles relaxes, your mind follows suit.
b) Breathing and meditation.
c) Learn calming techniques like massages, gardening, cooking etc.
2) Get professional help if you feel that the symptoms are taking too much of a toll on your daily activity. There are very effective medications, albeit with a few side-effects.
3) Adopt healthy eating habit and follow a good routine of exercises. Avoid alcohol and smoking.