I think it's important to try to remember any skills you did learn in therapy, for instance grounding techniques, relaxation exercises, and perhaps any of the CBT work you may have done to help you alter negative thinking patterns.
The only way to fully overcome these bouts of depression in the long term, (if it's caused by your past trauma) is to revisit therapy, and really work hard on your trauma and all those negative thoughts surrounding that.
However, in dealing with depression, one of the best things you can do is try to look after yourself and be kind to yourself. Make sure you have a healthy routine and force yourself to stick to it. So that means having a good and regular sleep pattern, avoid substances such as alcohol, nicotine and caffeine. Try to eat regular and healthy meals, and then give yourself some 'me-time' to do things you enjoy. And also try to involve both time in the outdoors, and a bit of exercise, even if it's just a short work in the fresh air.
I know how hard it is to do any of those things when you feel depressed, and it would depend on how deep your depression is as to what goals you set yourself. I know that, in the past, I've been so low that even just getting out of bed and showering has been a big ask. So depending where you are with you depression, start with smaller goals and build up. And remember to give yourself a pat on the back for everything positive that you do!