I suggest pushing on (with frequent small breaks as you need) or setting it aside for a while.

I had to read The Road for class and we were discussing it during one of my most anxious moments during undergrad; I was applying to graduate school and despairing about the options. I couldn't read more than 20 pages at a time before I had to go for a long walk and shake the shroud of apocalypticism off me for a bit. It was a good book, and I don't regret finishing it, but wow, was that miserable.

So just decide for yourself if you want to push on. (No shame if you don't; The Bell Jar is amazing, but it'll be around even if you wait a year.) If you do go forward with reading, try to build a routine out of it: read what you can and then do a self-affirming activity, as simple as walking outside in the fresh air or drinking your favorite tea.