Hi Viola,
That intense and I can understand that this might scare you. But it is in some ways also very cool, I think.
I had somewhat similar experiences with writing fiction myself. Some (not all) of my characters and stories have a tendency to be very independent and sometimes I feel more like watching instead of crafting a story.
Partially I explain this experience in the following way. Plural brain seems to have developed a general ability to create identities which act/feel/think independently of each other. Some of these identities got a stronger connection to the body or our life and were probably created by our brain because of intense problems, which were perceived as not possible to solve/escape by other means. These are the ones I would call alters.
There are others who are completely bound into a fictional world inside of our head and maybe even initially created by will. In most cases they stay there and they have no influence on the rest of us. Like... they don't seem to be be interested to use the body and with very few exceptions they don't even interact with anything outside of their world. But they use the same blueprint by which alters were created and therefore can develop independently in their world.
I think, what happens sometime is, this "breathing some life into them" you describe. We have alters who had a strong influence on fictional characters of me, as far as I can tell. Alea for example was also the name of a fictional character with very similar character traits, before I knew that Alea existed as an alter. Their life is very different though, since Alter-Alea identifies with our outside life and Fictional-Alea lifes in a cyberpunk world. Still... Fictional Alea shares his body too with other identities too

And there are some mixed case like Fae, who lives in his own fictional fantasy world with a lot of stories going on, but his own story has connection to our outside life (in a complicated way which I still can't grasp completely, because of some inception-like dream-in-a-dream layers) and he has some interest into the outside life too.
Short version, before it gets too weird: I think that human brains are capable of a lot of weird stuff and there isn't a final answer to the question, if fictional characters with a lot of agency on their own, are actually also alters.
I wouldn't be too scared. If the are and are interested in your outside life they will say hello at some point, I guess. But maybe they are just happy to life in their own world.
If writing is something you like, please don't stop it!
- Autumn