OCD. I have a touch of it myself and it can result in the same endless “perfection hunting” you are describing.
Before starting a project make a timetable for all phases…and respect them.
I use LW and over at the forums there, they did a speed modelling challenge every week, (I think it is still going amazingly).
That is very good for getting you to “just do it” as there was a set 1 hour time limit to model, the objective was to get something out, on a subject. Not 100% accurate, but so you knew what it was, some of my attempts were, ahem, basic. 
Another thing to consider is, a lot of pros simply do not put that level of detail in, as others have said, if it looks right, it is right. If you do not see the back of something, why waste time modelling it? Are those screws really going to show up in the render? No? well, image map / bump / fake them.
It’s been said by others already, but stepping back to see the bigger picture really can free you up, though I do suffer from a bit of OCD in that area myself. 
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… As others have already mentioned, set yourself hard time limitations and cut yourself some slack, give yourself time to develop, etc. With all the experience you gain from making a larger quantity of nice, but short of OCD models, your work will progressively tend closer to “perfection”, and at a faster rate.
And, if something just HAS to be perfect to the micron level, for a technical reason beyond personal OCD, I model it in a CAD package! Your guitar body is perfectly suited to CAD - draw the 2d profile sketch on a plane, extrude the body into a solid, cut away from the body or move the surface control points where necessary in the more “organic” areas, and finish up with some lovely variable radius fillets - finished in a couple hours, and accurate enough to be sent to a CNC if you want. Want to add the neck, bridge, tuning machines, etc? Same process. I haven’t tried it myself yet, but maybe check out the recently released, free, direct modeling (non-parametric) CAD software Design Spark Mechanical. It looks like an “essentials” version of professional CAD software Spaceclaim, which I’m quite familiar with and which I’ve found very easy to get to grips with. It’s not as powerful as a history-based parametric CAD modeler like Solidworks, or even a dedicated NURBs surface modeler like Rhino, but hey… the price is right, the learning curve is much lower, and maybe it’ll be useful? Anyhow, just thought I’d throw that out there.
Cheers,
J
Try Solidworks, fast, easy to learn, quite precise, quick predictable results without those “interesting” quests of topology cleaning, after applying some holes in subD mesh) It suited well for guitars, engines, gears, cars and other hard surface stuff
Solidworks is also $4000…
Yes, but it’s capable of solving complex engineering tasks) the guy who started this thread obsessed with precision (so as I), and I think he is wasting time with artists tools, because they are designed mostly to produce pictures, movies, toy figures and other entertainment stuff which are not needed to be functional. In Solidworks you can concentrate on design and functionality without thinking “how i need to proper cut my piece of geometry, layout those cuts to retain the shape and get a clean result”