Until this week, I've let Elizabeth's statement "We all love to instruct, though we can teach only what is not worth knowing" in ch54 waft past me without working out what it referred to.

Here's what I've got so far. The instruction is of Jane, about the extent of Jane's own feelings for Mr Bingley. It's not worth knowing this time, because Jane will be just as happy making the discovery at her own pace. It wasn't worth knowing back in ch21 (she was gradually led to hope, ..., that Bingley would return to Netherfield and answer every wish of her heart), because it meant that Jane's hopes got dashed a little harder than they might otherwise have been.