You don't sound borderline based on your description of your symptoms. I understand the whole wanting an identity thing, and not wanting it to change and worrying about faking. I've had some of the same, since I got my BP II diagnosis in January after telling the psychiatrist that I thought I was BP II and she agreed, so then I sometimes worry that I talked myself into it. But looking back at some journal entries, I was pretty clearly hypomanic in August, and when I told my best friend about the diagnosis, he was like, "yeah, I can totally see that", which made me feel better. And the Lamictal has been helping a lot, which is really what matters.
Try using the mood tracker and journalling feature on here. I know it's hard to imagine being anything but being depressed when you are depressed, and having a written record of your feelings will help you remember, or at least see how you were.
I know I should just be happy that the therapy and lithium are helping. And I am. But there's this nagging thought in the back of my head that says I'm not really bipolar, which leads to either I'm faking it or am actually borderline or some other mental illness, and I don't want any of that to be true.
I will tell you this bud,
Don't worry about it for now. Just live your life and appreciate that the Lithium is working for you. I never had to question mine much but I did have trouble in the beginning accepting it. I went into treatment depressed and took an MMPI. A couple weeks later I became manic, took another MMPI, and there was so much of a difference between the two along with the obvious mania that there was no denying my ibipolar type on.
If you wish you can get a second opinion and that may shed some light on the subject but the diagnosis isn't really that important right now unless you are filing for SSI or disability.
Bipolar is very hard to diagnose and a good dr will not do it without plenty of proof gathered to substantiate the diagnosis. My dr didn't even let me know I was bipolar until 2 wees into a very severe manic attact. Although they may not have diagnosed you yet, it is likely that they have a vvery strong feeling that you are and that is why they perscribed you the Lithium.
Hope this helps