I don't think she let's herself believe that he loves her. There's a point just after she hears that he has gone to the Lees (sp?) where she forces herself to see all of the attention he has paid her- not to mention that whole scene after the fire - as just kindness on his part. She knows how dangerous it can be for her to fall in love with a man she cannot have, and tries to see everything as though he's just a very kind master. Yet, she has to force herself to look at things like this. She's very intuitive, and her intuition about him says he loves her- although she never tells the reader this. She just hints at how she felt that they were of the same kind- and compares his reactions to Blanche with his reactions to herself and concludes from these that he doesn't love Blanche.

Then there's the fact that he has to tell her several times that he is serious. I don't think that the idea that he wanted her to be his mistress was an issue for her before the proposal. And just afterwards, she is more concerned that he might just be infatuated.

What I've been wondering about that scene is wether or not Mr. Rochester decided to propose just then. It could go either way. I'd say 'yes' but there are times when I think perhaps it wasn't so planned afterall.