Last updated: 7/15/23
(Disclaimer: this is more for the systems who care about their individuality/have an internal structure where individuality can even be a thing. This advice might not apply to some systems, and that's okay. Disregard if so!)
I feel like a lot of advice I would give to people wrt plural stuff could be summed up as "don't overthink it" and "you are not your personality" and "just let yourself exist."
I've lost count of the number of times I have seen people ask stuff like "lately I've felt more caring/easygoing/etc but I'm Supposed to be the Aloof/Anxious/Etc One(tm), does that mean I'm actually a different person/am fusing with another headmate/etc?" But the thing is: people don't have static personalities. People don't have personalities that are unchanging and can be neatly described and fit into a box. People change! You can start off anxious and become easygoing. you can start off aloof and become caring. Heck, you can be anxious and easygoing, aloof and caring. Thats Character, Babey.
Also, how we act and feel and think is constantly influenced by our surroundings. We act differently depending upon the things that are happening at the time, and we pick up the quirks and traits of folks we're close to. So yeah, if you're crammed into a brain with someone else, you're gonna pick up some of their stuff! That doesn't make you any less you, because what makes you you is not your collection of stuff but the fact that you exist, and can collect stuff to begin with.
Sometimes, you actually are someone else, or are fusing with someone else, or whatever. But it's not gonna help you to fret about the possibility when what's happening can be more easily explained by normal existing-as-an-individual things. You might ask, at what point do you consider the possibility that you're fusing/splitting/whatever? Well, it's the same advice I'd give for questioning plurality in general: when it begins affecting your life to the point that you need to explain it in order to do stuff about it, and fusion/splitting/etc makes it make sense in a way that Normal Personality Stuff doesn't.
tl;dr if singlets get to do it then you get to do it too ("it" is "existing without scrutinizing themselves for every little change or inconsistency")