Being Erica/WMG

The reason
Thus
 * More than likely  by 3x12, "Erica, Interrupted".
 * Sort of by Episode 4x08, "Please, Please Tell Me Now".

Doctor Tom is The Doctor.
And I didn't even say "X is a time lord" ... ("facepalm") Just think about it. He is a DOCTOR! He travels through TIME AND SPACE! His office is BIGGER ON THE INSIDE! (my theory is that his office IS the doorways that Erica walks through)
 * He also carries a water pistol (Episode 3x01, "The Rabbit Hole), just like Ten.
 * Alternatively, the Doctor was the original therapist.

Erica's voice overs are things she's saying to other people when she's a therapist.
The voice overs seem to speak in an older more experienced fashion and almost in a more teaching fashion as if she's telling people things.