What Fumi needs is a better sense of self worth. Getting worried isn't going to help.
If sensei's the type to fall in love with another person that easily then he's not gonna stay with her regardless.
Trust is important, too.

@Purplelibraryguy kotatsu smell funny tho

Actually I think stuffing your laundry under the kotatsu is a briliant idea. Your clothes would be nice and toasty warm, mmm.