i would also recommend checking into this:
http://www.ncees.org/Exams/Study_materials.php?exam=FE
those sample quests & solutions guides seem to be more indicative of typical questions on the FE - the FERM is generally held to be overly 'robust' in terms of difficulty of sample problems; but thats not a bad thing, as overly prepared is a good thing.
I hate to break it to you, but the double integrals are covered as part of multivarible calculus that most first year engineering and physics or math students would have learned in their first year of college.
Higher level math means topics such as partial differential equations, algebraic analysis (no not algebra of the type you learn in 9th grade), differential geometry, complex analysis, etc..
Then again, you should know the FE better than anyone since you did take it six time, that's about 5 times more than most people.
First let me tell you some factors why I failed and took it 6 times, for so many people in this board laugh at me, they dont have any clue why it happened to me.
1. I was educated BS Civil and MSc Engineering in the Far East. We are not educated to a level where the focus was passing the FE exam was the target, not like the ABET degree holders, they dont have that much excuse if they failed as much as like me. Our system was focused on Civil Engineering major subjects and our local board was based on this.
2. I graduated 15.5 yrs ago.
3. Some courses like Materials Science was never introduce in our Engineering curriculum, even Probability and Stats wasnt included, I learned this in MSc Engineering.
4. Our Thermo and Electricity wasnt that in depth we didnt reach Cycles and AC circuits.
5. I live in place where I have to do only self review, no PPI review or any review classes are held.
So in general it was a struggle for me, it took me years to grasp everything what the North America covered in their engineering course. I would say I learned it the hard way, I have posted my several study approach for the FE exams somewhere on this forum, feel free to read it, it might benefit you. Two things I would give you a recommendation.
1. Never give up, in the end youll be victorous, and I made it.
2. revise your strategy all the time you fall down and look back why and what went wrong with your study approach and develop good studying techniques.
good luck you can do it.
Steel Man