If this is you then you'll know that over-thinking doesn't really work out in practise, you soon hit a speed and efficiency plateau that becomes extremely frustrating. There is a solution however and that's to turn the brain off sometimes and just swim! Be warned though, if you're habit of over-thinking, this is not as easy as it sounds.
1) It allows no space for your body to find its natural stroke style.
2) It tends to add dead-spots to your stroke and kill its natural rhythm and timing, which of course is also a part of your unique stroke style.
3) The mental pressure makes you rigid and tense as you try to be perfect. Of course this only becomes worse as you get more frustrated.
A Variety Of Stimuli
The key to developing your swimming is to provide a range of experiences or stimuli to your body. Focused stroke correction is important but so too is swimming with the brain-off for a while with no pressure. When you do this just enjoy the experience and feel the natural rhythm of the stroke; you'll often find something clicks or you become aware of something that was holding you back.
Aside: This is how kids swim and is a major reason why they become so fast (it's embarrassing how quick some 12 year olds are isn't it?). Junior squads receive plenty of coaching but kids don't have a huge concentration span and their intellectual capacity isn't fully developed so for much of the time they turn their brains off and just swim.
Try this neat little set and see what happens. Repeat the set 3, 4 or 5x through:
4x 50m drill with 10 seconds rest between each
(choose any drill you are working on, for instance sculling, kicking on your side, unco.)
200m continuous steady swim with the brain switched off - just feel the rhythm!
If you find it hard to switch off during the 200m then it's very likely that over-thinking is a problem for your swimming and is holding you back.
There Is No Perfect
The truth is that there is no one perfect style that suits every swimmer. Trying to match someone else's idea of stroke perfection will frustrate most swimmers as it simply doesn't suit their body type, personality or experience.
Instead of trying to be perfect, your goal should be to let your body evolve the style that suits you and make the most of that style by refining it. Give yourself the right mix of stimuli - including swimming with your brain turned off - and you really will reach your potential in the water.