From one mystic to another, I think you might enjoy the books Journey of Souls and Destiny of Souls by Michael Newton. They deeply explore this idea of knowing particular souls across various lifetimes, the diversity of relationship types we might have with them, the lessons we work on together, etc. You might feel a lot of familiarity in those ideas.

That said, while I'm on the fence about The One, I do believe in soulmates and (probably) twin flames. I just realize that I define them in a less toxic way than most of society does, and I think that's why those terms work for me. A "soulmate" is simply a soul you've known across many lifetimes and have significant business with.... like you describe your mother or your sisters, for instance. A "twin flame"... well, that's trickier to define, but I do believe there are people we meet who fit (the healthy side of) that bill. What I don't subscribe to is all the dysfunction that some believers try to project onto the label, such as the idea that a twin flame is "meant to" hurt you profoundly or that the bond is meant to be dramatic, all in an effort to avoid admitting that they're in a very toxic relationship and need a therapist (or an escape plan).

As for "The One," I like to think that this only exists to whatever extent it is failsafe: that there are some things destined for us in a lifetime, and sometimes those destined "things" are actually people — and therefore, for some of us, a given lifetime is indeed meant to bring a very specific person as a life partner. But I don't think "The One" is the kind of thing that people need to pressure or blame themselves about. If we're "meant to" meet someone specific, then there's a larger purpose involved that impacts more than just us, and the universe sets up to make sure it happens. But if we're not destined for this in a given lifetime, I don't think we can really "miss out" on it.

I also believe that we don't have to believe in these types of connections in order to find them.... which is perhaps the most beautiful part of all. :)

Just my (oversimplified) two cents. Thank you as always for the thought-provoking read, Y.L.