Actually, I have a correction. I've edited the post above to reflect it. On the purl-side, use p2tog for the decreases on both ends.
But definitely count your rows first, to make sure this is necessary. Look for the first place where two stitches overlap, on the left side of the knit-side of the work. The sitch (or "V") above those first two overlapping stitches represents row #1. Follow the stitches on that row to the middle, to avoid the decreases, and continue counting up from there, and count all the way up, including the stitch on your needle. That's how many rows you've completed. To count your decreases, you can look for those rather obvious diagonal lines that pop out a little bit, where the two stitches overlap with the decreases you've used. You should have 17 of these lines on each side, at regularly spaced intervals, if you've done a decrease on all 34 rows. It sounds like you've definitely missed at least one. No big deal, if it's not obvious to your eye.
Also, I should note that if you use the quick fix I've mentioned, it will change the angle of your decrease line for those rows. Your decreases are all lined up in a perfect straight line. These will cut in at a sharper angle. But since it's at the very top, and near the neck, I suspect you'll hardly notice it, and if you do, it should be okay looking because of the location.
KnittingHelp Queen Bee
"There is a vitality, a life force, an energy, a quickening that is translated through you into action, and because there is only one of you in all of time, this expression is unique. And if you block it, it will never exist through any other medium and it will be lost. The world will not have it. It is not your business to determine how good it is nor how valuable nor how it compares with other expressions. It is your business to keep it yours clearly and directly, to keep the channel open. You do not even have to believe in yourself or your work. You have to keep yourself open and aware to the urges that motivate you. Keep the channel open."
— Martha Graham