Book an appointment with the doctor and say how you feel and that you can't cope at the moment [that's the hard part]. You should get signed off then and get a note to give your employer.
I will have to go to the doctors soon and I'm dreading it!
I got dismissed from the mandatory charity job last week. It got too much and the manager told me to leave straight away, so now I won't get a reference and might lose the JSA for 13 weeks. My advisor says I need to see the doctor again to decide if I'm unfit to work. I said I was fine before and that the SA was milder now, but it's really not.