I am really sorry that you have to cope with this. My fiancé has first hand experience of this! I can go from laid back to completely irrational in the next minute! I have a lot of health issues to contend with but that doesn't excuse my outbursts!
You just have to be there but if it's taking a toll in ur health then u maybe just have to let it go. Depression is a thing that consumes ur whole life, I have been dealing with it since I was 12. U don't even realise how badly u are hurting those around you.
Keep In touch. Ang xx