Well on the plus side, having done it for 18 months already, at least you know you are more than capable of handling this.
On the negative, dealing with all this while suffering from depression must be really debilitating, so I really feel for you.
My best friend has bi-polar and during depressive episodes she has days where she literally can't get off the sofa. She's described it as feeling as though her limbs are made of lead and that she's looking at life through a gauze blanket. It sounds horrific and she has my undying respect for forcing herself to function while feeling like this. If this is how you feel, you have my respect too.
I hope I don't cause offence, but you sound a little defensive, like you're expecting people to tell you 'it's not that bad' or that you just need to 'get on with it'. I don't think you'll find anything other than sympathy and support here, but unfortunately it is true that the only person who can improve your situation is yourself, although hopefully with a little help from the ADs, your Dr, your RL friends and your posts on here. It's human nature to want to offer advice along with support - it's not mean to devalue what you're feeling but to help. I don't think anyone would disagree that being you is really hard right now and I'm sure everyone hopes it gets easier.
When DS3 wakes, is he up for long? I got round night-waking by co-sleeping, but I appreciate thats not a solution that would appeal to everyone and only works if the child is looking for reassurance and isn't fully awake.
I've got to go now but I'll check back in later. I'll be thinking of you.