Basically, another way of saying you're having a living nightmare, ie you're in a really shitty situation.
Today's been AWFUL. I thought my English Literature exam was in the afternoon, but it was actually in the morning. I missed the exam, and now I've been told I've been held back. It's like all my exam-anxiety nightmares came true! I'm truly having a mare!
Small "mares" are usually caused by physics practicals, but larger ones are usually due to women.
The Ultimate Mare: Going out with a fit girl for two years, splitting up while living with her, sleeping with her friend, sleeping with ex again a few weeks later, pulling another of your ex's friends a few weeks after, then telling your ex the gory details including the fact that you did not play it safe with either and that she may now have got de baad AIDS.