I agree seek professional help if you can. There could also be physiological reasons that triggers an episode of depression, especially lack of B12, iron or D-vitamin.
Developing good routine is not an easy thing for people that are deep inside a negative spiral, but it will help limit the episodes. Good routine is like a spell, it reestablishes your commitment to each day and provides you with energy. This is the same whether you have depression or a bout of sadness.
Depression is not just a sad spell. Depression is a medical condition. Seek medical help.
(But yes, all of the things above are good things to do in any case, for anybody.)