I comfort eat when I'm down but can relate to what you said about how you feel, and about not eating or doing anything. Mostly because when I get down, really down, I stop doing things I normally love to do, or things that I ought to do (like take my asthma medication daily). It's self-sabotaging negative self talk that does it, and most of the time I'm not even aware of it.
Sometimes I just stay still, really still, cos then I know I am safe. I know that these moods pass - they always have and I've had them all my life. Giving myself permission to feel them, let them 'be', was important for me.
I used to feel guilty all the time (still do a lot of the time) when I'm not 'happy'. Who teaches us that we have to be happy all the time, especially around other people, including family?????
The tip about exercising is spot on - exercising really works to fix mood swings. I find that taking two omega 3 capsules, and a zinc/magnesium tablet a day helps as well.