It is true that we all do it differently, though I'm sure we all have a core, have someone who WAS before the damage made us split to survive.
I wish I had something profound to say that would help or reassure you and I don't.
You are giving your BF Jon a huge gift by believing him. I've been married 28 years and my spouse can't believe me - it weirds him out too much.
Please just keep talking here. At some point maybe you or you and Jon will find someone to help you with the DID. I will say this, I have a friend who's been married over 30 years and her spouse accepts her alters and I think He rocks for just accepting and working with her to get along in marriage WITH the differences, not in spite of them.
You are not imagining things, you are not freaking out. I'm sure the core said those things too you and maybe will communicate more with you as you go along together. It doesn't seem like there's anything to fear. Hang in there, Kendall