If you are concerned that you are suffering from mental ill health, you should visit your GP. You could also go to see your occupational health adviser if you have one. They may then refer you on to a specialist mental health professional e.g. psychiatrist, psychologist, counsellor.
The most common mental illnesses associated with stress are anxiety and depression.
Most importantly, see your GP for advice and treatment.
For all cases of ill health, if you think it would be helpful, you can request a referral to see a specialist counsellor or psychologist through your GP, or your workplace may provide counselling for you. You could speak to your Occupational Health adviser or your manager to find out more about this. Your GP or counsellor should also be able to provide you with further information, including contact numbers for support groups and crisis telephone numbers.
If you do not feel strong or confident enough to speak to someone on your own, you could take a friend or family member with you when you first go to see your GP or psychiatrist for a diagnosis.
General information can be found at Mind, Rethink, Saneline and Samaritans.
The Samaritans have produced a booklet called Coping with Stress [130kb] 
which you may find helpful.
Finally, the BBC also have a mental health section on their website that may provide further helpful sources of information and links.
What if I also have a physical illness resulting from excess pressure or stress at work?