What does “force oneself ” mean in the sentence,“Dickens has been forcing himself. Bleak House was forced; Little Dorrit was labored”?

Forced, in this context, means that the work wasn't written out of some burst of inspiration or the pleasure of writing, but out of necessity or contractual obligation. Dickens didn't want to write any of those books, but did so anyway - and their quality, the article implies, suffers for it. That's what forced novel means in this context: a piece of literature that didn't flow naturally, wasn't fun or enjoyable to write, but rather a chore, possibly even one involving hard labor, that had to be done.

The article's author then extends the metaphor further with the dug out as with a spade and pickaxe. If writing these books was such a chore, like digging in the mines, than adding more digging metaphors would stress the hard, repetitive nature and mindlessness of the task.