Difficulty level of 501 Tesuji Problems

Recently I have started going through the book 501 Tesuji Problems, and I am finding most of the problems obscenely hard, I spent an hour and a half on one page last night and got 3 out of 9 right (around problem 85.) See http://www.godiscussions.com/forum/s...ead.php?t=5663 for an example of a problem. I have checked the GD product database, and LPDavid and Slarty both suggest that the book should be possible for a player that is close-ish but not yet a Dan. Sensei's Library does not have information for this book on its useful "Problem Book Grades" page.
Just in case any of you were wondering if I am torturing myself in the name of getting better at Go, this is not really the case  I really like the Mastering the Basics series, especially the excellent Making Good Shape, and I appreciate all the explanation diagrams and the cool way that similar problems recur helping to crystallize the tesuji pattern in my mind. 

Two most common questions:

1: How hard have others found this book?* Is it that my reading is unusually poor for my rank? I found it comfortably lukewarm (I would read it before I sleep every once in a while; just from those bedtime readings, in the end I got through about half the book and began reading something else from there). I have never read Get Strong at Tesuji, James Davies' Tesuji, or the Kensaku Tesuji dictionary, so no comment there.

2: Is it worth doing a book that I find so difficult, I mean, will I learn things? IMHO, no. It is much better to go through problems you are comfortable with and getting through many of those done than to crawl at a snail-pace over problems that fly over your head. Solving problems should be comfortable and enjoyable, not stressful and torturous (of course there are a few exceptions...).