Oh Blossom - what can I say? Been there, got the Tshirt and come out the other side, possibly wiser, definitely less patient and accommodating.
My DP has cycles of depression, we're actually overdue for one, and the toll on my mental health has been immense. I do sort of get what you're saying about the depression being the issue, not the person, but, and it's a 'but' not an 'however', 3 way relationships don't work, ie you, him and the depression. You will play 2nd fiddle to the depression, will be expected to compromise for the depression, possibly require treatment for depression yourself and can't blame anyone because it's the depression. Coming of medication without medical support is such a red flag. Depression can be a life long condition, requiring life long treatment, just like diabetes, high blood pressure etc.
There was a very interesting thread yesterday from about a wife whose husband was refusing to follow health advice for diabetes, had been for some time and now was starting to see adverse effects. She's at the end of her tether, and is contemplating leaving.
Only you can ultimately decide but trust me when I say, as much as I love my partner, I will not be able to support him through another cycle, without his active engagement in treatment. My health, my mental wellbeing will not be able to. I have taken the decision that I can't run myself into the ground. If others judge that to be a failing on my part, then so be it.
Your DP needs to understand he has a major role to play in managing his life long condition and if he can't do that for himself and for you and any potential children, please do not feel obliged to sacrifice your health alongside his. And this is on him, not you. he needs to be the active partner in watching and maintaining his mental health, seeking additional treatment as and when. Not you.
I had to give my DP the tough love talk several years ago, he gets his act together and gets treatment or I walk away. I think you need to do the same. Don't get all romantic about this - hitching your life to someone else's is a major step and both parties need to fully commit, being distracted by depression is no excuse.
Sorry if the tough love approach sounds harsh, but I have lived it and it's no bed of roses