What is Hester coming to realize is the true sin she has committed? Why would Hawthorne consider this a worse sin than her sin to Dimmesdale?

I think that Hester Marvels at the fact she married Chillingworth in the first place. Her union with Chillingworth had nothing to do with love as opposed to Dimmesdale who she truly cared for. The truth of her adultery was that she loved Dimmesdale and the gift of Pearl that she loved even more. The same can't be said about her actual marriage to Chillingworth.