Mr. Age, the problem is not exactly what you said, but that you linked what you said to SeatBelt's comment. You made it personal. It may not have been your intention, but that is what was perceived. The impression is that you linked anxiety, depression, and shame to SeatBelt's post. You may have had good intentions, but it was not taken as such. It was, indeed, poorly worded.
Misunderstandings happen, and in fact are quite common without the benefit of non-verbal communication. However, the interesting thing to me here is that you didn't simply defuse the situation, but instead, decided to dig deeper. One simple post could have straightened out the misunderstanding.
You are correct that you, on some level, are not responsible for anyone's reactions or responses to what you say.
However, you do not get to choose whether a person's reaction or response to what you have said is right, wrong, or "tripe". By labeling SeatBelt's reaction as "tripe", you have indeed made and acted upon your own judgment based on what SeatBelt said as a assumption about your post...which is what started this all off in the first place.
You went further still by doing the exact same thing based on what Duchess said. You acted off of assumptions. An educated person should have known better than to continue to dig one's own grave expecting to actually get back to the surface...or perhaps I should recommend a good Logic 101 course refresher?