Thank you for asking such an interesting question and sharing your difficult life situation. It is heartwarming to learn that you are determined to get well by becoming a student in psychology. I hope you enjoy pursuing the subject and understanding more about your own psyche.
From your brief account, it looks like what you have at the age of 15 was 'double depression', that is a major depressive episode superimposed on a long standing, low grade depression of at least 2 years' duration. In order for the chronic depression that you outlined to be considered 'dysthymia', one will need to have low mood for at least two years. This would mean that you had had low mood since age of 13. I wonder if that is correct. Was there something stressful or traumatic that happened to you at around that age? If so, a large part of therapy may have to focus on that stressful or traumatic event so that you can come to terms with it and move on with your life.
I also wonder if the initial diagnosis of double depression may just be a tentative one and the clinician was actually intending to revise it when there is more information available or as the clinical scenario evolves. Hence it will be good to seek another opinion and get the condition thoroughly evaluated and re-diagnosed. In psychiatry, the diagnosis may change over time when the clinical picture changes or when more information is available to the clinician.
For chronic depression, it will be good to exclude some underlying medical problems like thyroid disorder. Also, there are newer antidepressants available in the market and these can also be carefully considered in your case. Oftentimes, a patient with depression may not respond to a particular antidepressant but he/she may do remarkably well with another medicine.
It looks like you did not have a second episode of major depression after 15 and just have low grade depressive symptoms for many years. Now that you have insight into your condition, you can play an active role in preventing yourself from developing depressive episodes. A good book to read will be this title, 'Overcoming depression (third edition) - a self-help guide using cognitive behavioural techniques' by Paul Gilbert. The book includes easy-to-follow, step-by-step suggestions and exercises to help you understand your depression and lift your mood.
Proper stress management strategies will go a long way in helping you prevent depressive episodes. Learning to prioritise your tasks and challenges, exercising regularly, having 6 to 8 hours of sleep a night, are measures that will help.
Take good care of yourself.
