Do chemo treatments for breast cancer get progressively better or worse?

Yes & yes: Some side effects like neuropathy, fatigue or blood count issues can be cumulative with subsequent cycles. Some have a perceptual component and patients may perceive them as worse (if they dreadfully anticipate them) or better (as they get used to them and know what to expect). It varies a lot.

Usually worse: Patients usually feel more side effects as chemotherapy progresses. This is a general rule but you might be different. Positive thinking and stress reduction are helpful.