Hi, Tana. You say that you got your EPUB files for "Nook for My PC". Does that mean that you purchased these books from Barnes & Noble? Unfortunately, B&N uses a special-DRM encryption on their EPUB files that doesn't work with the Sony ereaders. Some of the B&N free ebooks (and also samples) do not have this encryption which is possibly why you say a few of them do work.
I can understand your frustration. Most likely your ereader is fine. Do you have the Sony Reader Library installed? You can use it to setup an account, register your ereader and access the Sony Reader Store. They have a category called "Bargain-Priced" that has free ebooks. You could purchase a few of these $0 books as a test-case just to make sure your ereader is fine.
You probably just need to buy books from compatible sources, like the Sony Reader Store, Kobobooks, Borders, booksonboard. You can also download public domain classics from places such as here on MobileRead, feedbooks.com, gutenberg.org. There is a stickey thread on this forum that is all about where to buy books.
https://www.mobileread.com/forums/showthread.php?t=11052
Hope that helps and Welcome to MobileRead!