How do I get the pieces of my sweater to stop curling up?
There is a sticky on this subject.
St st always curls, which is why most sweaters usually start with ribbing, garter, or another none curling stitch pattern. If the sides are curling that will be fixed when you seam or with whatever finishing is required for the sweater.
And if it’s to be seamed, that will tame a lot of the curl.