Stepmom breaks up with her fiancé after 8 years of her 14-year-old stepdaughter treating her like dirt: 'My life is better now that his daughter isn't around me.'

She tried so hard to connect with her stepdaughter, but only so much rejection. 

It's so difficult to accept that some relationships don't work out because of people outside of the relationship. This person could be anyone from a jealous ex to an overbearing mother-in-law, to a child from a previous relationship who cannot be cordial with their parent's new partner. When the relationship itself is good, one might be reluctant to end it because others object to it. Every couple and every situation is different. A married woman might be able to accept her mother-in-law's possessive antics because she lives 3000 miles away from her and her husband, and they only see her once a year. But she might not be able to accept a stepdaughter who lives at her house full-time, who is committed to being cruel towards her despite her best efforts to connect with her emotionally. 

The hardest part of situations like this is that you can't force someone to like you. You can be a literal angel sent straight from heaven who is nothing but nice to your stepdaughter and sensitive to the problems in her life, but she can still dislike you and be committed to making your life miserable. It's up to her biological parent to take steps to figure out what's going on with her, and figure out if they can continue a relationship that makes both their partner and their child despondent.