If I slide the page, the textbox loses focus, which means the cursor isn't there anymore; so I have to touch the textbox again, but then the cursor, once again, ends up in another place entirely than intended. If I pinch-zoom it, the textbox is now small enough so that I can see all of it, but then I still have to place my finger on the spot I want to edit. The problem is, I cannot read what I wrote, OR place my finger accurately enough on the place I want to edit.
An no, using a pen is not an option with the screen I have. It's one of those which only work if you touch them with conductive curved shapes such as fingertips or, say, metal spoons (yes, I tried that; it works, but is of course far too impractical.)
I could flip the phone to be in the "long side up" format, but then the keyboard takes half of the screen, and the textbox the other, which, once again, makes scrolling around neigh-impossible.