Hi Wren.
Similar story here, second pregnancy with short cervix diagnosed at 23 weeks, put on bed rest and placed a pessary.
I went in at 33 weeks with PTL, they gave me the steroid shots and started me on nifedipine. I stayed on nifedipine every 6h just like you for one week. It made me feel pretty crappy because of headaches and low blood pressure. I stopped after 34 weeks - there's just no credible medical reason to stop contractions after that gestational age. If baby wants to come, and you've done all you can to prepare, then let it come! Better out than in. We usually say:"we might not know why they want to come out, but they do".