Importance of planning

...and I mean really planning - not just doing and seeing where it gets you. (which mostly is my methodology when it comes to knitting without a pattern. "I can do this - I don't need a pattern. This is easy." ...Right)

Some weeks ago I showed some knitting projects that I had on my needles. There was this "I'm going to be a big& cozy jacket" - thingy as well. I should have known better than to just knit on and on. It's not that I didn't do any progress. I managed to knit the back&fronts&sleeves & pockets (it's a top-down raglan - so the knitting part went well) and I was just about to begin the cable panel that I could envision running along the front edges and around my neck - and would be some 12-15cm wide and it would look so good... (see how big the gap between the left& right front is? That's for the panel...pockets look good though)
So far so good. Except - when I was happily casting on for the cable panel I noticed that I only had a skein and some yarn left for the panel. Right. Not good. (I totally thought of having 9 balls of Rowan Polar - but was obviously wrong - I only had 8 to begin with)
How can I make such a mistake? the panel with cables will take approx 2 balls of yarn (that's my rough estimate) I'm screwed. Really cannot figure what to do next. I rather not use a different yarn to the panel - and I cannot see it in two different yarns either (one of which would be the Polar) Damn!!