Trust me, I live with this same feeling. Unfortunately, it's not two souls inhabiting a single body, but one soul chained to another, and they share the same body. Try to understand here, two souls in one body is not a bad thing, in fact, you can learn from one another. But when you, as a person, lose sight of who you are, that's where the trouble lies.
To deal with something like this, I'd suggest sitting in a quiet room with a candle of choice. Light the candle, close yours eyes, and focus on your immediate surroundings. Try and sense if something else is there, or if it is all internal. If all internal, then try to communicate with the soul/spirit/whatever it is. Remember the purest form of you, who you are, who you were.
Eventually, a split will take place, it's not instantaneous, and will take time (so I suggest doing the meditation regiment often), but you'll find that another person has emerged.
Acorlei, the spirit I deal with, is a very unpleasant fellow, and we share the same body meaning we both hear and speak through it (verbally, sometimes loud enough for others to hear). If this happens with you, try to keep it all mental, unless you have a good excuse to speak out loud (mine is voice acting practice).
I hope that answered a question or two. Spirits be with you.