Sweet spot sessions should not be too hard to finish without adjusting the intensity.
Could there be a chance of some physical reason for your tiredness, possibly overtraining or something like that? For me personally, there was a time I had a hard time getting basically any session done well, and when I went to see a doctor, we found out that my blood ferritin level had dropped for some reason (possibly overtraining), and that caused the tiredness for me.