Cayda, do you write your rows down as you do them, then cross them off when you do the matching piece?
Never just trust measurement.
I was taught to knit at four years old and I'm now fifty years old and I have always written my rows down, because I put all my shapings on there too.
For instance, on the front piece, when it tells you to "divide for neck" (or words to that effect) you'll know exactly which row to do it on.
When you have finished the left side of the neck, you can cross the rows off to ensure you shape the right side correctly.
For instance, when I need to decrease one stitch at each end of a sleeve, next to the row number I write "D1EE" on my sheet of paper, so that I know what I've got to do on the matching piece.
If it's decrease one stitch at neck edge I write "D1N".
For decrease one stitch at armhole, it's "D1A".
It really does help you to know exactly what you're doing.