This is not the easiest passage to read for a learner of English, so kudos to you for your attempt. In simpler English, the passage is: He dropped it in the box and heard it fall. Using his "interesting" ability to watch his own stupid action even though this ability is unable to stop him from committing the stupid action, he guessed, as he continued to walk, as to what might happen after taking the first concrete step to becoming once again Ethelberta's lover.

I am wondering why you are reading this particular novel. It is not well-known and it contains difficult language. Most native English speakers would find it difficult and old-fashioned. In other words, I think that the only native English speakers who would choose to read this book are scholars or fans of Thomas Hardy who want to read everything he wrote, no matter how obscure and unapproachable. In addition, perhaps it is very much above your level of English. This is not meant to be discouraging, but reading a novel that perhaps 0.0001 percent of English speakers have heard of is perhaps not the easiest way to improve one's English.