If you've been on two livery yards and only come across one bitchy person you're probably not doing too badly! She can only upset you if you let her - see her behaviour as what it is and refuse to be drawn into it. Either say to the yard owner that you're considering leaving because of the harassment and see what if anything will be done about it (but you do have to be prepared to follow through so have options beforehand). Or rethink your yard visits and be working in the school or out hacking or not there at all when she is.
Stay yourself with everyone, don't get defensive or start avoiding people you think might be on her side, that gives them evidence of your not being friendly. Be the person you are and be seen being friendly, lies are best confronted simply by living the truth openly regardless. Be carefully polite to her too, so people see if there is a problem it is in her mind or on her part, and nothing to do with how you actually behave. Kill the rumour with kindness and bright carefree enjoyment of your hobby which is after all why you are all in that place isn't it!