Are you sure that you are only doing the 6 yarn overs? Are you accidently making more yarn overs, perhaps?
I would count as I go
k2tog 3 times will give you 3 sts
yarn over and k1 6 times will give you 12
k2tog 3 times will give you another 3 sts
I suggest placing a marker every 18 sts and counting each st pattern, every 18 sts and that way you can see what you are doing, okay? It's really fast to count your sts, too. I do it with almost all of my lace knitting and it becomes 2nd nature.
I would also count the none lace rows, too, for a bit, until you get the pattern going okay