It can be confusing what the difference is, because there is often overlap in what both of these professionals do. They both specialize in mental health. Their training is different, though. Also, the way a psychologist and a psychiatrist work can be very different.
Boundaries are everywhere. There are physical boundaries, like the door to our house, the lines between cities, states, or countries, and even our physical bodies. There are also emotional boundaries.
Depression is complex, multi-faceted, and different for different people. Still, most of the time there is a deep, terrible emotional darkness with embodied isolation and turmoil.