It sounds to me like you're extremely frustrated with how your mind is working right now. That's natural. I'd be frustrated, too (and I was) if my mind wanted to do different things every day.
Hell, my mind STILL wants to do different things every day. My triggers for "changing identities" are usually stress, meeting a new group of people, or moods.
It's pretty common for people to project their feelings onto their therapist. You're actually one step ahead--you realize that you're doing this.
You also realize how differently you can act at times. For many people, even THIS can be a difficult process. So, consider yourself to be one step ahead!
A bit of advice (check with your therapist): don't automatically attribute "mood swings" to an identity shift. When you're depressed, you're likely to act differently than when you're happy. Also, people naturally act different in different situations. One of my favorite quotes is that you're not the same person when you talk to your mother that you are when negotiating with a hooker.
It's only when there's a lack of a pattern to your actions, and how you see the world, that it becomes a problem.
Sometimes, meds can help with depression. It also helps to take stock of how you usually act. If you're not happy with the pattern, you can always move forward and learn new habits.
I was lucky to find a calm person, who is my best friend, fiance and, to an extent, role model. Don't get me wrong, I don't take every word of his as gospel. But his presence in my life does help me be more stable. Since he rarely has mood swings, and since he isn't the drama type, it's easier for me to maintain my pattern of thought and behavior - two things that I consider to be a major part of my identity. He also has other qualities that I greatly respect, and want in myself. Sometimes we talk, but other times it's just nice to know he is there. It doesn't always have to be about therapy. Sometimes, the calmness and traits of another person can rub off on you just by being around them enough.
Try to find an anchor point. It can even be something on your keychain, your purse, or something that you won't forget to come in contact with every day. Something that will calm you down and remind you that you are your own person. An anchor point can also be a person--but avoid copying them. Just be mindful of the qualities they have that you want, and let yourself absorb them naturally.
You might have been attracted to the Narc for his confidence or his carefree way. This is just a guess. You might have felt good and stable when you were with him. NPDs can be very manipulative. It might help you to realize that you wouldn't have been able to act that way, if it wasn't in you to begin with. Different people bring out different qualities in us. Although it's easier to be "you" with some people vs. others, remember that it's YOU who gets the final say.
Try starting your first journal with "I am...A student, a music lover." I think that it's ok to write "negative things," i.e., vent, in your journal, but PLEASE avoid putting yourself down. If you get the urge to write (FOR EXAMPLE): "I look bad," try to counter it with something positive, and something you can control: "I'm going to make an effort to take better care of myself--dress better, keep my nails and hair neat." Or, (another example): "I'll never stop feeling this way." Counter that with, "I've learned a lot about myself already, and I'm taking steps to get control of my moods and actions."
A routine helps, too. For me, it's tempting to "control" when I sleep, and then stay up all night. But think of controlling things positively. Eating healthy meals or snacks at approximately the same time each day, sleeping at around the same time, and doing other activities on a schedule help me feel more stable.
Just food for thought. It helped me, but you might be different. You are your own person, after all.
Do not take my advice before talking to your doctor/counselor/other professional. Depending on where you live, you may be able to find free, confidential care. Most importantly, sometimes your shrink can be wrong. Get a second opinion.