Wow, that's a great response - thanks, everybody! There seems to be quite a spread of approaches to building, and some good ideas I can try. Some of you (RealG, Tankboy51) don't actually plan, and build what takes your fancy at the time, but stick more or less with it until it's done. This is a more disciplined approach than mine, up to now.
Others (e.g.TB) have a number of builds on the go, so when your enthusiasm for the one you're doing at the time dries up, you have a range of others to choose from and can find one of them that which is different enough to re-ignite interest. This is a ggod idea, and something I try to do, but on a small scale (pun absolutely intended) - 2 or 3 kits on the go at one time, but some still get put aside & left untouched for ages. I don't think having a lot of alternatives would work for me, I'm afraid.
One or two (Bish, MrMike) plan well in advance and more or less stick to the plan, but I don't have that degree of self-discipline. Tojo72, you plan ahead, but not as far, which is what I have also tried, with limited success e.g. the BoB build.
Some of you (Keavdog, Modelcrazy, Dodgy) have hit on the fact that GBs provide an added incentive to manage builds, which I think is very significant. I have taken part in only two GBs so far, but both got finished - although one did sit in the Doldrums for a while, I got it done when the deadline loomed.
So, from your kind responses, I have decided to try to -
1) Plan, but keep it realistic i.e. limited to my own attention span, and try to tune out the voice in my head saying "this is boring now, just leave it" (thanks, Eaglecash!)
2) Keep a (small) number of interesting alternatives going, as back-ups in case the voice gets too loud!
3) Participate in more GBs
I've also hit on a way to limit the impulse builds (and the impulse buying that goes with them). I intend to start making a list of the kits that I suddenly absolutely, definitely must buy, instead of just reaching for the credit card. I'll look at the list in, say, 6 or 12 months' time and see if the enthusiasm for them is still there.
I feel a lot more positive now I've got this sorted out in my head, now I just have to make it work. I'm really grateful for all of your input, thank you all very much!