Jarvis
I wouldn't consider myself the sharpest tool in the shed, far from it!
But the I think the key to passing instruments is understanding the Electrics involved in Aircraft General. Gyros, Pitot acceleration errors and all that are easy enough, there are plenty of feedback questions to prepare yourself. Its the electrics the Capacitive and inductive reaction are the tricky ones that you must understand completely, very nearly tripped me up.
(remember C I V I L? in Capacitive induction, current leads voltage, inductive volt leads current? or something!)
So my advice is, out with the highligter pen and highlight like there is no tomorrow!!
Keep your chin up, with the right atitude and determination you will get there, just make sure you are as best prepared as you possibly can be, do not waste another sitting if you are not at least 90% confident (You should never be 100% confident.)
This may not be much use to you but just thought I would my tuppence worth