Hello and welcome Christina, and any others who might be there!

It sounds like you've certainly come to the right place. Do you know if you have a diagnosis of DID in your past? Because I've found that often helps, at least when it comes to therapy. I'm very sorry that you've been bouncing around so much throughout therapy, I can relate to that. I had 4 therapists in the span of 3 years.
Although you might be nervous about bringing it up to your therapist, it's crucial to voice concerns and thoughts to her. She can't help you if she doesn't know what's wrong. If you're able to, would it help to bring your SO to your therapy session? He could help verify signs or symptoms that you have, and even those that you aren't aware of. I find that therapists are more likely to listen when they have some sort of proof or verification.
My psychiatrist was skeptical and didn't believe my concerns. He thought I was "self-diagnosing", when really I was just bringing up the fact that I matched so many symptoms. This made one of my alters, Kat, very angry, and I actually lost time and switched without knowing it. She ended up yelling at my psychiatrist, asking him questions like "you think I'm fake?". My aunt was in the room at the time, so was my therapist, and my aunt was very scared at how different I was. (Kat's voice is deeper than mine, her jaw juts forward in defiance, and she usually has an angry scowl on her face). Even after that my psychiatrist put it down as "possible DID". So sometimes we just have to trust ourselves. Only you know what goes on in your mind, how you feel, what happens within you, so trust yourself and what you know. Don't let anyone try to place doubt in you if they're skeptical or if they doubt you themselves.
Oh and if she's a therapist, no one should sound "crazy" to her. It's her job to work with different people that have different troubles, problems, and conditions.

Even if you're beyond her expertise, she should at least be open to what you have to say. Perhaps you could try printing out symptoms of DID and circling ones that you experience? Or even write about the symptoms you experience? That might help her to see where you're coming from, and might help her to understand DID a bit if she's not familiar with it. Maybe, if you're comfortable with it, you could print off some of your blog entries. Let her know of the ones you don't remember, and highlight important parts that stick out to you (like arguing with yourself or something). Perhaps even write about what emotions you feel when you talk or argue with yourself.
I talk and argue with myself all the time, aloud or inside my head, and different voices always answer back, either aloud or inside my head. So you're not alone in what you're experiencing, and you definitely don't sound "crazy".
Perhaps, to help get more "concrete" evidence, your SO could record you in some way when these different Christinas come out? Either by video or even simple voice record, if the voices are indeed different. It might help you feel more confident in your suspicions and might help your therapist to see what you're talking about.
You could also trying communicating with these different Christinas and seeing if they communicate back. Like writing to them (on paper) and seeing if they write back, seeing if you remember when they write back, or seeing if you can learn more about them via talking to them, either aloud or in your head. The more you know about yourself and possible other parts, the more you'll be able to share with a therapist (not necessarily your current one) and the better chance you'll have of getting the proper help. I found that the more I knew, the less doubt I had of myself, and that allowed me to stick up for myself and what I knew instead of allowing doctors to brush me off or misdiagnose me. (For the record, I'm still undiagnosed since the last psychiatrist never got a chance to change the "possible DID" diagnosis, and after that I was switching therapist and psychiatrists too often to get a formalized diagnosis on that topic. I am diagnosed with rapid cycling bipolar disorder, post traumatic stress disorder, depression, and anxiety though).
I hope you find this place helpful, and that you're able to progress in therapy. Feel free to post anything you're comfortable about posting. And again, welcome.
~Cassandra and Company