Needing a bit of help with depression

[TealElephant]

Hi ladies, im having a bit of trouble deciding what to do. Me and my SO have been together a few years now, we have a beautuful 2 yo girl and he has a cheeky chap (almost 7 YO) from a previous relationship who we have shared custody of and im 27 weeks pregnant with the next cheeky chap to add to the mix :)
I've just been diagnosed with 'moderate to severe' depression (first time I've ever heard a scale used by a doctor!) And had an emergency referral sent to a psychiatrist.
Now to get to the point, ive suffered with depression and anxiety for years and while everyone close to me is aware of this, I've mafe a conscious effort to sheild them all from it. My SO is the main one I'm worried about, he has never had to deal with my depression, never seen the full extent of my anxiety and hasn't had to deal with me getting help before. He's an impatient, foul tempered sod (not harmful or anything, its just who he is) and I dont think he'll understand or know how to react, so I dont really want to tell him. But I know I have to because I cant deal with this alone anymore. Any advice on how or when to broach the subject with him? Or what to say? Or any encouragement or advice at all really please!
Thanks for reading my essay lol