HUGE deep breaths, and any relaxation technique that makes you fell comfortable.
If you are constantly feeling anxious, panicky etc, try this. Stand with your feet shoulder-width apart, hands by your sides, shoulders relaxed (yes, relaxed!! As relaxed as you can, anyway). Take a deep breath in through your nose, hold it for 2 seconds, breathe out slooooowly through your mouth. Repeat 3 times. On the 4th breath in, slowly raise your arms above your heard in time to the breath. Hold the breath for 2 seconds, then exhale as hard as you can (imagine you're in labour!) and as you do so, bend forward at ths waist, still with your arms above your head, as though throwing yourself forward, until you are as close to touching your toes as you can manage. Repeat as many times as you can.
Moving your body in this way will help the flow of oxygen to your brain, helping increase your seratonin and making you feel calmer.
Try googling yoga techniques for similar things to try.
Could you go to a local yoga class? I know money is tight, but your health is important and classes only cost a few pounds.
Also, if "every waking minute you have a seething beast of cold fear and panic writhing in your stomach" you do need to see your GP to talk about it. There are some anti-d's that have an anti-anxiety effect. Might be worth investigating. There is no shame in taking them, and no shame/failure in admitting that you need help. You night be depressed, despite thinking that you're not. (One of the symptoms of depression is denial!). I'm sure you feel that you can't afford to fall apart - I know that feeling only too well. But facing up to a problem, and dealing with it, is a brave and strong thing to do - doesn't matter how you face it or deal with it, as long as you do so. Which you are doing. I am full of admiration for you; just please don't deny yourself something (e.g. anti-d's, etc) which could possibly make things much, much easier for you.
I remember your post earlier this summer - you sound as though you are dealing with the practical side of things brilliantly, but please do get help for the emotional side.
Hypnotherapy can be wonderful. It is expensive though. Could someone help you with the cost of this? You would only need one or two sessions. I've written a post on this in Mental Health if it's any help ...
I hope this helps you.