This is a common state of mind and all but the most starry-eyed superteachers will experience it now and then. But such feelings can be seen in a positive light; indeed, they are a healthy part of managing a career - they warn us that the old motivations have worn thin and that new challenges are needed.
The danger of this mindset comes when you fail to respond. If you don't do something to fill your half-empty glass, just staring at it won't help. A classic sign of depression is a feeling of hopelessness about the future and an inability to control your own life.
Jean Heslop, headteacher at Cliffe Hill and Mixenden primary school in Halifax, says: "In the end it's up to you. You must take responsibility for driving your own career because nobody - not your head, not your friends, not your colleagues - can decide what you want to do."
Put more brutally, nobody will be impressed by a headstone inscribed: "Here lies John Smith. He could have made more of his life but his headteacher wouldn't let him."
So what options are available? Well, why not talk to your school about promotion, new or even reduced responsibilities? Try to think beyond the normal career structure.
Sabbaticals: You may have noticed a rash of headlines a year or so ago about a new government drive to offer more teachers time off. You may also be wondering what happened to them. In fact, there is now a scheme to offer a few hundred experienced teachers in challenging schools sabbaticals of up to six weeks. These allow them to take part in research projects, carry out community work or other schemes related to their work. To qualify, you must work in a school in which 50 per cent or more of the pupils are eligible for free school meals.
But be warned. David Clegg, Newcastle's principal adviser, says these opportunities are still quite rare. Only five Newcastle schools from the 32 that qualify for the scheme took part this year, although he anticipates many more over the next year. Half your battle will be to convince your head to part with a valuable member of staff in the face of unrelenting pressure to improve results.
DIY sabbaticals: However elusive official sabbaticals may be, those working in parts of the country with severe teacher shortages or who teach subjects such as ICT and modern languages may be in a position to improvise their own career break. Voluntary Service Overseas, which offers two-year placements abroad, says some recruits are being offered deals by their local authorities to return to their jobs after a period of working abroad. Even without such a deal, there's no need to worry about not being able to find a post when you do decide to return to the UK.
Study options: Another good idea is to set aside some time to do a course. The National College for School Leadership runs courses designed to be followed alongside work. You don't have to be a head or a deputy to take one. Middle management staff can embark on the National Professional Qualification for Headship and there are plenty of related schemes aimed at less senior professionals. The Open University offers a range of home-study modules which count towards Masters in Education degrees. These require a commitment of between eight and 16 hours a week. Alternatively, if you don't fancy such in-your-face education theory, you could think about a more applied course in art history or maths.
Other ideas: If you're finding it difficult to express yourself in your work, you might also consider volunteering, or you could set up an extra-curricular club within your school. Of course, such ideas require a commitment of spare time - a commodity which many teachers find they simply don't have.
If your blues still won't lift, you could always follow the advice of the psychologist Oliver James: "Go and buy yourself a juice extractor and put half a pound of carrots, some pears and beetroot through it. Then drink it - that should make you feel a bit better. And if you're seriously depressed, go and see your doctor and get some Lustral."
Challenging schools sabbaticals - www.dfes.gov.ukteachersprofessional_development
Voluntary Service Overseas - www.vso.org.uk
National College for School Leadership - www.ncsl.org.uk
Open University - www.open.ac.uk
Volunteering - www.do-it.org.uk