Slow Learner

So.
I generally think of myself as a pretty intelligent gal. I have a pretty decent IQ, got good scores on the SAT, graduated with a relatively high GPA... But like so many nerds, I am apparently lacking in common sense.
My current problem is directly related to knitting, a hobby which I enjoy very much.
You see, when I start a new project, big or small, I tend to buy just one skein of the yarn that I will need for the project. Then, when that skein runs out, I go back to Michael's or JoAnne's for another skein, and so forth.
Well, I'm about four rows away from finishing a baby blanket I've been intermittently working on for over a year, and I have run out of yarn. And I can't find it anywhere. Apparently it's discontinued.
What's really bad is that this is not the first time this has happened to me! I've had to abandon 2 or 3 works in progress or rip them out and start over because of a yarn shortage. I should have learned by now to purchase all the yarn I need up front!
Now, I will have to try to find one more skein of the fiber I need online, where shipping will probably cost more than the yarn itself. And since I haven't saved any of the wrappers from the previous skeins, I'm not sure what color I need. The online pictures are not very true, so it could be either "seaglass" or "light thyme." I'm pretty sure it's "light thyme," but I'll probably order one of each just to be safe.
Arghh - one of these days I will learn!!