Just to confuse you a bit more.
The difference is in volition.
When I say I am going to do something. I mean I kinda want this to happen; it �s my intention.
But when we use the present continuous, the listener can just understand that this plan is on our diary/schedule. There is no information whatsoever as to who scheduled or planned it.
Example:
I am going to meet my boss tomorrow. (I planned this, or I played a role in planning it.)
I �m meeting my boss tomorrow. (I don �t want to tell you whether he wants to see me or I want to see him.)
The present continuous tense is therefore more secretive than "to be going to."