It's only SOME modernism and post-modernism that's difficult (some is not - proust, for example, i found so lovely). I guess things written in middle/early modern English, too, are difficult for us today. Everything else should be sweet.

I think it's just inexperience. Like anything, you get better with practice.

As people have suggested, try reading a summary, but experiment with reading the summary before you read the section of the book.

Also many people hate this because it destroys books but if it helps it helps - try reading with a highlighter and highlight key phrases. It helped me understand marx in college so maybe it will help you too.