Unfortunately, major depression does tend to run throughout the course of a lifetime, particularly if it has not abated by the mid-to-late 40s. That is the bad news... the good news is that it is a cyclic disorder, so the severity will ebb and flow throughout the lifetime. In other words, if it truly is major depression, then it is not a condition that sticks around with no relief forever and ever... it will hit you hard, hang around for awhile, and then ease off. The cycle should be well-established in your life by now, so at the very least, knowing the general parameters of the cycle should offer you a bit of control and an ability to brace yourself for impact when you know you will face a particularly difficult stretch. For example, I have had a number of clients over the years who have had the worst time dealing with their depressive cycles right around the time that we change the clocks in the Fall. Knowing this allowed me to help them prepare for the down cycle.
It also looks as if you have tried several medications to control the cycles. Seeing that you have been on Diazepam tells me that you have also suffered from anxiety or sleeplessness, which will both magnify the depressive symptoms. It may be that treating the anxiety may help to relieve some of the depression, although your doc would know best, XXXXX XXXXX has a full knowledge of your case.
But to answer your question directly, I would say that yes, depression is going to be an issue that you will struggle with for the rest of your life; but you can take steps to help lessen the duration, depth, and frequency by making some simple lifestyle changes. Exercise is at the top of the list because there is a large pool of research and anecdotal data that tells us exercise (even alone) can have a profound effect on depression. Too, diet and sleep corrections will make improvements. Getting yourself healthy in body will help to keep you healthy in mind.
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