Thanks for your reply.
I don't think that the symptoms you have noticed are withdrawal symptoms because withdrawal symptoms only appears once the medicine in question is abruptly reduced in dose or is stopped cold turkey and you haven't done so. These could also not be because of side effects of Venlafaxine as you are taking this medicine since last few years.
There could be two possibilities in my view.
First of all these symptoms " I would describe it as a very sudden and short vertigo. It's just for a split second, it feels as if the ground shifts, or as if on a boat. A swaying sensation, if you will. But it disappears just as suddenly as it appears. As I said most often when I'm walking, and when I turn around quickly." could be because of some infection /problem related to ear or nose and if yes, you need to consult a E.N.T doctor for this problem.
Assuming that there is no medical problem and these symptoms are because of worsening of anxiety
, the other possibility is that now Venlafaxine is not able to control your anxiety symptoms properly.
I am saying this because you are noticing these symptoms since last few months and once you tried to lower the dose, the result was the similar severity of symptoms and worsening of anxiety.
If this is the case, adding an antianxiety medicine such as Klonopin for short term should solve the problem.
I hope this helps.