It's probably not you, she just may not be mature enough as a person, and when she sees you it reminds her that your dad had a previous marriage so it makes her feel insecure....Just be strong, pray, and don't rehearse it in your mind of you being wrong or what you could've done differently just let it go and move on with your day.
Maybe it's a little harsh but you can also pray to God to set your stepmom straight....God knows how to break pride/set people straight and if you ask him to he just may and change your stepmom from a cold heart to a real one...
Don't let people who suck drag you down...know you're better than whatever they say!