I have been playing this chord progression that I came up with (which is almost the same shape constantly), but I want to understand why it sounds good.
The progression is: DminAdd9 C Bb6 A7
It probably sounds good for reasons other than music theory: you recognize it from all kinds of songs, including "Hit the Road Jack." Your progression is the same, you just added a 9th in the first chord and a 6th in the third chord.
This particular progression is an example of what we call the lament bass. Also called a "step-descent bass," it's exactly what it sounds like: a bassline that moves down by step. Your progression is an embellished i–♭VII–♭VI–V, which is a very common lament bass progression. So common, in fact, it has its own name: the Andalusian cadence.
As for your added notes, note that they are chord tones of the next chord: The add9 above D (E) is in the subsequent C chord, and the 6 above B♭ (G) is the 7th of the A7 chord.
See also What chords should accompany descending bass notes in a scale? for some other harmonizations of this idea.