There's a great post here which will give you some additional resources to study for the ISSEP:
https://community.isc2.org/t5/Certifications/New-ISSEP-Official-Guide-and-or-training-for-the-March-...
At least two members from here (including me) used the list in that post to successfully pass the ISSEP fairly recently. A lot of them are NIST references, and, yes, they can be a tough read, but all the information you need to pass this exam is there.
Not sure, if this going to encourage you but it will hopefully inspire you... The ISSEP is the most exclusive of ISC2's certifications that aren't regionally focused, so it's very worthwhile persevering with so you can join the club!
Also some words of consolation... I heard from someone apparently in the know that only 30% of ISSEP exams result in a pass, so you're certainly not alone in not passing.
