Leaving the past in the past is a very good thing to do, unless it's creating problems for you in the present.
There are so many different things that can lead to depression -- as Daily Llama says, it could simply be wonky brain chemistry, and once you find the right medication you'll be fine, but childhood trauma is not an uncommon cause. If there seems to be no current reason for your depression and there's been a trauma in your past, it's probably worth doing some dredging to see if there's a link.
When I left my marriage, I didn't realize the impact on me of 9 years of verbal and emotional abuse. Ok, looking back now it's obvious that not many people could walk away from that unscathed, but at the time I was just focussed on moving forward and minimizing the impact of the divorce on my children. Long story short: we were living in Austrailia, I moved back to Canada with my kids, found a good job, bought a house, settled the kids into school and neighbourhood, and never really talked about my marriage or what I'd been through. It was in the past -- over and done with, as you said.
I believe that everything happens for a reason, however, and I think that the powers-that-be felt that I hadn't taken the learnings that I was supposed to take away from that experience, so they started heaping on the stressors until eventually I cracked and then had no choice but to go back and heal my old wounds. I suppose that if, say, my father hadn't taken ill and passed away, or some of the other stressors hadn't happened, I could have kept going the way that I was, but it really wasn't a life. More like an automaton, or the Energizer bunny. I'm still healing, and still learning, but I'm feeling again which is... well, ok, it's still a little raw and tender, but I'm rediscovering my authentic identity -- if that makes any sense.
Maybe your trauma and your depression aren't connected, but I think that it would be a good idea to find an experienced therapist to help you check.
And keep writing. It's as helpful to be supportive as it is to get support on this site, and we can all learn from one another's experiences.
Take care.