Thank you all for your replies! I know that maybe this isn't a result of any particular condition. It just doesn't feel quite right, and so I think there might be something deeper going on.

I did think about the maternal instinct thing. The problem is, sometimes I think of becoming a mother and it thrills me, and I realize how wonderful it'd be to live through the eyes of a child again, and get that whole experience through my child. But then, I often also think of it and get terrified! I don't know if I can handle that kind of responsibility. It's a forever thing, and is so permanent, and so important, and I'm so scared I'd mess it up, or couldn't handle it. I know to an extent, these are all normal feelings. Maybe my issue is I just think that I feel things too intensely and it takes over.

As for still partaking in some of those activities (like having homework, sleepovers, etc.) I just don't see that as a real option right now. I don't know anyone who would have sleepovers with me :P

And I'm working on seeing a new therapist right now. My last experience with one was really negative, but I do thing ultimately it'd be nice to have one. I had a kind of difficult childhood too. I have a feeling that might have at least something to do with my preoccupations now. But not everyone with a difficult childhood does this, so there must be something more to it. Is there anything else that has these kind of alter ego kind of feelings and fantasies? Or maybe there are different types of BPD that I don't know about?

I'm sorry. This stuff feels SO hard to articulate. I feel like I'm doing a pretty poor job of it :P I'll write more details about what I think I men later. But for now, thanks so much for your response!