Thanks NYCindie, I appreciate the reply, and the first half was really useful to hear. Unfortunately, the second half read like something my therapists have been saying for years, and while I understand some people need to hear this kind of thing, I've already heard it before.
I feel like I might be latching onto polyamory, but only because it's the first new idea I've had for some time. There is nothing else I can find within my life that is bothering me, and yet the depression stays, as it always has. My current relationship is better than anything I've ever experienced before (I found someone who actually believes that total and open communication is the answer, rather than just saying it), the stress at my job has settled now the company has grown and for the first time in my life I have enough money to not worry about how I'm going to afford to eat at the end of the month. My 20s have actually been the most stable decade of my life so far, but my depression is still here and nothing seems to be able to shake it.
I've seen the 'bring your best self to a relationship' phrase around on here, it's a good phrase. Sadly, I just don't think I know how to.