I think your first post shows an incredible amount of self awareness and you're clearly able to analyse your thoughts, moods and behaviour honestly and in depth. I'm sorry to hear of your suffering and a lot of what you describe resonates with me and is similar to how I experience bipolar. The inactivity, feelings of emptiness and desire to isolate myself that you relate are all classic features of my depressive episodes. On the flip side, the exuberance, astonishing energy and grandiose thinking echo how my hypomania manifests itself. However, for me, if not treated quickly, the highs often turn into full blown mania with psychosis.
I can definitely understand what you're saying about feeling like different people, depending on your mood, as when in an episode my behaviour and thinking are wildly out of character and this has lead me to question my 'true' identity.
You've made a great first step seeking professional help today. It certainly takes a lot of courage and is something to be proud of. I also had one session with my psychiatrist, when I was depressed last year, in which I cried solidly for an hour in despair at my situation. I was mortified at the time and beat myself up over it, but this thinking is characteristic of depression.
As hard as it is, try to remind yourself that your doctor's there to help you out of the distress- plus, I'm sure they've seen way worse!

My psychiatrist also gave the advice of trying to make myself keep to a routine/some activity, as when depressed I had zero motivation and everything felt like just 'going through the motions'. When deeply depressed, this 'routine' only really consisted of actually getting out of bed, making myself shower, eat regularly, take a daily walk, listen to music instead of sleeping etc. At the time it felt torturous making myself do anything, though I have to admit these small steps helped the depression from dragging me lower and lower.
I hope that with the right treatment from a good doctor, your suffering will gradually ease and your mood will stabilise. Over time, you can introduce lifestyle changes to help manage the illness too... The suggestion of a mood diary is an excellent one and I find minimising stress, plus regular exercise, a healthy diet and daily meditation helps. However, the right medication is the key to get me to a place where I can make those changes.
Last year, I thought I'd never get out of my deep depression, as it was just sticking and we had to trial some different medication combinations. In the end, a mixture of a mood stabiliser, antipsychotic and crucially, an antidepressant, did the trick and I've been in remission for over a year.
It might not feel like it now, but there is not only hope but every chance that the right treatment (i.e. medication plus therapy, ideally) will have you feeling like yourself again in time. Take care and all the best.