Emmie, there rarely is any reason you can put your finger on, far more often an accumulation of general rubbish. The thing with depression is you can't see a way out because it sucks you in, you have to just hold on and let others see that route for you. So much of it is because of a chemical imbalance in the brain, the sertraline and other drugs in that family work by rebalancing those chemicals. It takes time, but it does work, those drugs have worked for millions world wide, You are not any different.
If I can give you one bit of advice, stop resisting , the ' yes but ' I see so often in your posts is reinforcing the problem, I know how easy it is to get into that habit and it's a way of delaying taking positive steps. It's a contradiction but I am too ill is an excuse we use to stop ourselves believing we can be well.