I always see Anne as almost perfect, especially in terms of judgement, but to my mind her one flaw is that she has a real blind spot when it comes to her own self worth. I agree it is very sad that Anne imagines herself in this light. However the 'term glowing and bright in prosperous love" I think refers to what Anne imagines is the imminent engagement of FW and LM. In comparison to their fruitless engagement, Anne is imagining that there will be nothing to interfer with the prosperity of their love.