I did something called the "Lightening Process" at the weekend. It is not a miracle cure but does have a high success rate. I definitely feel better. They show you a technique which helps you change your thought patterns everytime you feel you are (what they call) heading for the "Pit".
You are constantly flooding your system with stress hormones which eventually makes you ill.
Disenchanted 3 - SO very sorry you are feeling so rough and like others have said this is not something you could prevent yourself. However having suffered from anxiety and depression in the past I know the feeling that you should be able to stop it happening. I think it is one of the symptoms of the illness - it's a bit of a deceitful illness because it makes us think we should be able to prevent it.
Depression and anxiety are very closelylinked and almost always come together. The anxiety is often the first symptom to show itself and the last one to subside. Your GP may prescribe anti-depressants and they can be helpful for a short time. Counselling may help too to get to the bottom of the cause of your problems. I know someone has talked of chemical inbalance but the problem is that there is no real agreement with drs about whether the chemical imbalance is caused by stress or somesuch or the other way round - a bit of a chicken and egg situation.
Do hope you will be feeling better soon. There a lot of self help books around which may be helpful. Don't forget that depression is almost always an illness with a beginning a middle and an end and you will feel better but it will take time. I say this because when you are feeling so low it is difficult to believe that you will ever feel better...........but you will.