Even considering just that axis, I'm not sure how one would define a spectrum that has low-grade symptoms on one end and manic episodes on the other. If anything there is a spectrum of affective states ranging from severe depression to severe mania, with most people at most times clustered around the origin point - and even at that, you can't map it to a line with two ends because mixed states exist.
Speaking to your specific case, and also for anyone who might be in the same shoes: suicidal ideation isn't the sole purview of depression or even of mood disorders as a whole, and I really wish the conversation around mental health would expand enough for more people (both patients and healthcare providers) to actually tackle it properly. Besides that I don't think you need to encourage anyone "not to be [you]"; you've found something that works for you, which is more than can be said of the healthcare system sometimes.
How would you actually do that? Is there some kind of home test available?
This sound like good advice, but I've kept it hidden so long I'm genuinely afraid that I'll have some kind of breakdown if I let it out.
I'm sure it varies a lot from person-to-person, but for me, I have a constant and profound feeling of sadness and futility, I often feel like crying, and occasionally consider ending my life (I almost fantacise about it, because I wouldn't have to feel this way any more).
One of my parents has manic depression, and a sibling also has clinical depression - I presume there is a genetic factor at play; I was brought up in a middle-class household with parents that loved me, and I seldom wanted for anything. Now, I'm a successful software architect with a wife, kids and side-business. I own my own home, I have 2 nice cars, and I have a high income. Yet I still feel that sadness, and often just don't see the point of living, or life at all, really.
Except for randoms on the internet, I've never told anyone about this. I've come close many times, but I guess the longer you keep a secret, the harder it is to let it out.
If anyone else is reading this, I'd enourage them not to be me, and to seek medical help. If you don't think you can speak to a doctor, I'd also recommend trying 5-HTP - I started 200mg 2x a day a few years back, and the change was incredible. It may have saved my life.