I relate to this so much. So at least I can assure you that you certainly are person and that you're not alone feeling this way.
I've found it manifests in different ways.
One is in the form of a belief. In my experience, this tends to be triggered by low self-worth, a feeling of inadequacy, and possibly loneliness. Another similar one, but a bit difference, is the lack of boundaries that you're describing, between yourself and other people. A severe lack of boundaries oftentimes lead to identity disturbance, feeling unsure of who you are, or as if you aren't a "real" person at all. And vice versa. If you're not sure of who you are, you're likely to have poor boundaries. I believe that some autistic people are prone to this, since we're fundamentally different, but many of us grew up not knowing why, and not receiving the needed amount of acceptance from others as well as ourselves to develop a strong sense of self and self-esteem, both of which healthy boundaries require. I know I used to not even understand what boundaries were or that I had the right to develop and maintain them. I was a people pleaser. You may have heard of the term "empath". People on the spectrum are typically known for having low empathy, but some of us are high in what's called affective, or emotional, empathy. I personally believe I am hyperempathetic in this area. What it means is that you're basically extremely sensitive to the energies, moods, and atmospheres of others. You lack a kind of "border" between yourself and other people. You're able to absorb pretty much anything, and may struggle to distinguish what is "yours" and what doesn't relate to you, in what you consume. Some people see it as a psychic ability. I've been told by a spiritual person that she could sense I took on energies around me and that I needed to perform a ritual in order to "close" myself up haha. She literally gestures zipping up a jacket. I like to view it more scientifically, as something that may be caused by my autism, or just the way my brain chemistry works. It can be an advantage. It can make you very caring, understanding, loving and insightful, but also a disadvantage. Especially if you're unaware of it, or bad at protecting yourself, your energy, and keep your boundaries.
Another way I sometimes experience this feeling of "forgetting" that I'm a real person is through dissociation. Depersonalization in particular. It's a very unpleasant and scary feeling. It makes you fear you're losing your mind. It doesn't necessarily imply that you're watching yourself from above or something. To me, it's feeling like my soul or consciousness is somewhere other than my body. I remember once I was walking, but I felt like I was floating above my head, and my body was moving mechanically like a machine. It can be looking at parts of your body, such as your hands, and feeling like they don't belong to you. It can be looking in the mirror and feeling panicked because it doesn't feel like you, and you start to doubt whether it actually is you for a second. I've struggled with this for years. My triggers include insufficient sleep, stress, anxiety, and depression.
Edit: Oh damn, just realised this post is from 2012. I logged onto this site for the first time in what feels like ages, and I've been thinking about this topic specifically for the past couple of days, which I thought was neat so I wanted to share. Maybe it can help others if OP won't see this, but if not, it was nice to articulate my thoughts. I see a lot of people making fun of this, and I get how it sounds funny, but it's actually a very real experience. Be nice to OP 