Planning - and keeping to the plan

I wonder what effect planning has on other writers? There's a continuing debate over whether writing prose is a creative art form or a craft, like chiselling a detailed pattern on a decorative wooden table. My first book, CUT SHORT, was an undisciplined outpouring of ideas, mainly because I wasn't writing it with a reader in mind. I never expected anyone to read it. I wrote it simply because I started and couldn't stop. It was great fun to write, and I thoroughly enjoyed the creative buzz. Those of you who follow my author blog may recall that when my publisher sent my MS to an editor, I had something of a wake up call. 'Your poor readers won't have a clue what's going on.' So I reworked my rather self indulgent MS into a more coherent form which thankfully went on to receive positive reviews and a lot of word of mouth buzz, resulting in two reprints in the first six months.
I did plan the second book in my series, ROAD CLOSED, but despite my best efforts, I still ended up making some very last minute changes to the MS. (Not in my publisher's good books at the moment, I'm afraid.)
Now that I have an agent, he has encouraged me to write a full synopsis for my third book, DEAD END. Problem solved? You might think so. But now I have to try and stick to my synopsis... and I've already had to make three major changes...
I honestly find the writing is quite easy. It's the planning that I struggle with.
Am I unusually disorganised or is this the same for everyone? And does anyone have any helpful hints about planning books? Any hints, tips or suggestions will be very gratefully received - preferably before my third book goes to the typesetters...
Leigh Russell