Which books have you simply been unable to finish?
I've given Ulysses a few goes but never got passed the first 100 pages. What books have you struggled with?
I've read some ball-breakers in my time - quite like taking them on tbh. Struggled with plenty but will always finish them - view it like training, you can have a horror of a session, but you'll probably be better off if you see it through and start again tomorrow.
Ulysses is up there but not the worst - my copy has a forward by Anthony Burgess where he makes the point that it's essentially a simple story, Joyce himself said all he wanted to do was give the Dubliners one good look at themselves. So while Stephen's voice is a challenging read, for example, the book as a whole is digestible. Plus the last chapter (Molly Bloom's soliloquy) is probably the most scintillating prose written in the twentieth century, so it's worth the trouble
Very tough reads for me would be stuff like William Gaddis or Thomas Pynchon - both US postmodern heavyweights. Pynchon has some accessible stuff, but
Gravitys Rainbow,
Mason and Dixon and
Against the Day are absolute monsters. All are very long and you've got dense, difficult prose on the one hand, married to impenetrable, crazy stories about the military industrial complex.
Gravitys Rainbow was worth it for me as it is an amazing novel. The other two were probably net negatives - some reward but not enough for a major reading effort.