I did a 30 day challenge this past December and about halfway through, I felt similarly. I had a couple of classes in a row where nothing went right. My focus was all over the place, I couldn’t balance, I had to sit out even the most basic postures.
My instructor said that Bikram yoga has the ability to break you down before building you back up to better than you were before. I pushed through those rough days and my practice was better for it towards the end of the challenge.
Your instructor has a point: some times the practice sucks. But the important thing is to keep going and approach each class with no expectations and see where each one takes you.