Hello essbee!
Swimming is definitely good, regular exercise always helps, and there is a rhythmic calming aspect of lane swimming that really chills you out IME
How about trying a herbal tea at bedtime (provided you can stomach these, I know they are not for everyone), or switching to caffeine-free Rooibosch tea from 5pm (again not to all tastes but I love it and it's a drink you can share with ds1 as it is OK to give children), or a mug of warm milk? There is even a sort with extra serotonin I think? Jersey milk? Seen it on the shelves in sainsbos.
It may or may not be having an effect on your two. But they know you are their mummy and you love them, honestly. My dh has struggled with depression over the past year (he is a lot better now) and I have been amazed at ds' acceptance of dh's bad moments, his tiredness etc. I am sure you are doing this already but explaining it's an illness, you are getting help to get better and it is not your children's fault will help and your ds is old enough to take at least some of this on board.