Thank you all for your replies! Grateful, I think you've got exactly what I was getting at, with realistic esteem.
I think that external forces sometimes, well meaning though they may be, make us feel that there is something wrong with us unless we are feeling good about ourselves all of the time.
As you say, sometimes feelings of low self-worth allow us to reflect to learn and make positive changes. For example, towards the end of last year, I went through a period of really feeling sorry for myself because of my weight. Whenever I saw myself in the bathroom mirror, you can imagine all the things that went through my head about how fat and useless I thought I was.
Then I had an epiphany - that I actually had the power to change things. I didn't have to be like this forever, or be this person. My 'low self-esteem' was actually triggering me into action, and I started on a manageable program of exercise and healthier eating, nothing drastic but just enough to make that change.
A few months on, and people around me have started to notice the change in how I looked. I hadn't even kept track of how much weight I'd lost, but the comments prompted me to do so, and it was nearly ten pounds.
This piece of information has also reminded me to not be so hard on myself and that if I want to make changes, I can make a difference. You'd think I'd be old enough to know this by now, but I think we all need constant reminding that we are not helpless and hopeless, despite what our depression may tell us.