Dude I really do sympathise as I have had a similar problem of late. I used to be like K, not thinking about work or taking it home but after a couple of personal problems at home and a change in management and workload it gradually crept up on me.

If you really are having trouble sleeping and living normally, go to your doc's. They don't necessarily have to give you medication but if you are having trouble they suggest that you self-certificate yourself for a week and then they'll certify you for and extra week or more if needed. During this time work are not allowed to contact you and must allow you to recover.

Also make an appointment with your HR manager and check policies regarding health and support. Talk to them about the problems of stress, workload and the fact that you have had to visit your GP. If they have policies they must stick to them (unless there are exceptional grounds like assaulting your MD) and if they have any regards for legality and morality they will normally go above and beyond their policy to help you. This may include giving you time off, reassessing your line-management and/or workload, phasing you back into work and at the very least having an occupational health advisor assess you and your position.

This is nothing to be ashamed or upset about. I work with a chap who encouraged me to do the above because i was becoming depressed and my boss didn't listen and piled more on. He is 1 of the nicest guys I know but due to having essentially a mental breakdown due to stress and reacting to late, he is now a shell of his former self. He was a stacked, extroverted ex-para who has turned into a timid, nervous guy who can barely complete a press-up. Scary stuff.

I think like K said, you have to talk to the right people before it's too late.