It sounds to me like Mr. D. thinks he exerts a LOT of influence over you. The words and manipulative actions he's used don't speak well of his boundaries or of his perception of yours. They also reflect zero respect of your girlfriend. He sounds, to me, like he's playing some alpha male game and making you a beta male in his pack. That's fine in certain social situations, but when it comes to your life, your goals, your decisions about who's important to you, You need to be alpha male.
I'd be interested in hearing what compromise you've come up with.