His constant proclaimations of decision and firmness of mind, IMHO, are knee-jerk reactions to being in Anne's company and subconsciously revisiting the feelings of rejection. He is saying all this to Louisa, but it seems to be meant for Anne, whether she is present when he says it or not. It's as though he is thinking out loud, calling Anne weak-minded and persuaded, he is so focussed on this quality as if trying to protect himself from being rejected again. He doesn't really seem to look at Louisa at all or discover how shallow she is compared to Anne, and her lack of wisdom with the constant jumping and her demanding to get her way seem to prove it. I think that if he was really LOOKING at Louisa, he might have discovered that he turned her into a spoiled bratt and would perhaps be more on his guard towards her. So, without thinking about what he was forming in Louisa, he persuaded her to be stubborn. She jumps off the steps as if it were some kind of test to see just how 'decided' she really could be!