Think of "tha" as "you" rather than "thou" - which is not used in modern English (even though it is derived from "thou")
"tha'lt nivver" is "you will never" derived from "thou shalt never" in the older form. (Some may see it as "you will not never" using a common, though grammatically incorrect, double negative)
So.. "It isn't horrid," he said, "even if you think it is. And you can't make it horrid. Don't fret yourself about loving me. You will never force yourself to (do) it. There's sure to be a bad nut in a basketful. You must take the rough with the smooth."
