i totally agree with madisgram, look at it this way, there is a raging house fire, three people are trapped inside, the fire department arrive, do they run straight in to help the 3 trapped people? ...no, they take time to protect themselves and put on their breathing apperatus, why ? ... because they know they can't help the others if they get overcome by smoke and need help themselves. you seem to be running straight in to help your boys with no regard to your own wellbeing. stop, take the time to prepare yourself, get you in a good place before trying to help others get there, it may even be easier as you will be then able to show them the way you took back from depression.
try to understand what triggers you, find stratagies which work for you, once you have found your strength only then can you begin to help your boys.

it is ok to look after number one (you) first, no one will think any less of you for doing so, it is a sign of maturity and common sense to do so, as too is asking for help, is there anyone or any charity/organisations who could shoulder your boys whilst you concentrate on healing yourself? this is certainly not a sign of weakness or that you are not a good mother, afterall a good mother would seek the best for her children even if it means asking for help or stepping back temporarily to heal herself so she can be there for them in the longer term picture.