Ok, you’ll have to forgive me if I’m wrong here… having not knit this before, I worked the pattern out in my head, visualizing as I went along.
I think this was your mistake… I think you skipped the row in bold text.
…Cast off 12 stitches in moss stitch at start of next row. Decrease 1 stitch at end of next row by knitting last 2 stitches together. Decrease 1 stitch at start of next row by knitting first 2 stitches together. Increase 1 stitch at end of next row by casting on 1 stitch after last stitch. Work 1 row without shaping. Cast on 13 stitches at end of next row (25 stitches on needle)…
Does this sound right? I believe it should looke like a U when finished…