Hi holsten,
just another person who is on your side. You sound very together to me for all you are coping with. I went through a phase of being unable to leave the house, months off work, suicidal etc.
Looking back I was very very unhappy with my life. I also had a relationship with someone who did all the taking and no giving. I wasn't happy at work either (dead end job, no incentive to perform better).
If I get down now, many years later, my first inclination is to not leave the house, and sit around doing pretty much nothing. So absolutely well done to you for the aerobics, bringing your DD out and doing the housework.
I have an emotional bank account theory - what you take out of your emotional bank account must be put back in or you end up empty. Sometimes you can be overdrawn for short periods, running on your reserves, but eventually you will go bankrupt if you don't get anything back.
You mentioned you had cancer - huge drain on you. You are a lone parent - also can be a drain (I know kids can put in what they take out but you are still the adult one, ultimately responsible etc). I think you actually hit the nail on the head yourself when you said this depression is your body's way of telling you you are just not happy.
Have things happened in your life you didn't get a chance to grieve? Have you always, always kept the show on the road, ploughed on, got things done, looked after the DCs? Even when you had cancer?
I am very glad you are going to your Mum's for a break. It sounds to me like you rarely get one. Is there any way you could move closer to your Mum's? Is your eldest DC living away at uni?
I hope I am not offending you. It's just you sound so strong and capable, I just feel you are completely worn out. You need kindness and dinners cooked and a break from responsibility. 