How can I overcome dreadful insomnia?

When you diagnose a sleep disorder, the first thing to do is assess what is causing it - is the issue biological ( caused by the brain, hormones etc) or psychological (caused by learning, cognition, the mind etc). It sounds like your work pattern has essentially taught you to not sleep at appropriate times.

This is not Insomnia - this is called Sleep Phase Delay Syndrome. Essentially you have the ability to sleep, but years of working shifts means that your circadian rhythm is not in sync with the world.

First thing to try - sleep hygiene. You can find these on the NHS Direct website. It's about preparing yourself to sleep - try everything. These are tips like having a bath before bed, not eating before bed, reading a book before bed etc. Try them all and see if any of them work for you.

Being that this is clearly a psychological issue - you may want to consider Cognitive Behavioural Therapy. This will be carried out by a Clinical Psychologist - you will need your GP to refer you. It is also available privately. They will teach you to dissociate from any negativity towards sleep, and change the way that you think about sleeping.

Chemotherapy is not a suitable option here - I have never prescribed drugs for this problem. They are highly addictive and have terrible side effects.