If you frequently have difficulty with reading visually, you might want to consider an all-audio ebook and audiobook reader instead of a mainly visual device. There are still lots of ebooks that you can read on the computer that you already have instead of on a stand-alone portable ebook reader.
I am legally blind, and even though I have found ways to read ebooks efficiently on my Sony Reader, I still rely heavily on audiobooks. It sounds like you would qualify for service from the National Library Service for the Blind and Physically Handicapped. It's not just for blind and mobility impaired people, they also serve people with what they call "organic brain dysfunction," reading problems due to strokes, seizures, brain cancer, or head injuries. I don't know the extent or exact nature of your problems, and I'm not a doctor, but if you have a disability that prevents you from reading standard print books even part of the time, you can get service. You get books on tape in the mail, and over the next couple of years there will be a transition to digital books that will be delivered in the mail on cartridges or download from the website. Your state affiliate library will supply you with the player you need for free (the digital players are not available yet but will be rolled out over the next few years, and veterans have 1st priority when players are available) and the books also come to you and back to the library for free through the mail. You can also purchase a Victor Reader Stream ($349) and get started with the downloadable books right away. The Stream also supports Audible.com books, 3gp, aa, brf, flac, html, mp2, mp3, mpg, ogg, rtf, spx, txt, wav, wma, xml. It does not have a screen so all of the non-audio formats are read using text-to-speech. In addition to NLS services, the Stream can also be used with NFB-Newsline and Bookshare.org files. I have no affiliation with Humanware, but I am a very satisfied Stream user.
http://www.humanware.com (sells the Victor Reader Stream)
https://www.nlstalkingbooks.org/talkingbooksform/ (sign up for NLS services)
http://www.nfbnewsline.org/ (Newsline is sponsored by the NFB but is available to anyone with a print disability. You can also get newspapers via your telephone or email)
http://bookshare.org (Bookshare provides scanned books to people with disabilities under a special provision of copyright law, annual fee is $50 but this can be waived if you volunteer or receive a scholarship)
If you have any further questions I'd be happy to help.