18 sts and 24 rows to 10cm square over st-st using 5mm needles.",
The description of tension helps you to make the size sweater that you'd like. If you're getting 18sts/4inches on a small test square of knitting called a swatch, then the size of the finished shrug will be as the pattern predicts. There's good information on swatches and gauge here
(about 2/3 down the page). If you want, you can measure your tension on the part you've knit already.
The neck bind off looks good and the neck shaping (under the Divide for Fronts section) but you don't have the sleeve bind offs or maybe not in the right place. It's disappointing to have to rip out but if you go back to the start of the sleeve shaping adding sts for the sleeves on each side and then proced to the Divide for Fronts you should be ok. Come back if you're unsure and post more photos so we can see how it's coming along if you'd like. Take heart, we've all had to rip back on projects and so we understand how hard it is. It'll be worth it in the end.