Dear sweet girl,
Here's the trick, build up your self esteem to the point that you can set and enforce healthy boundaries, this way you can keep those at a safe distance who would normally lower your self esteem. By that I mean do things you love. Dress the way that makes you feel good, join activities you enjoy and spend time by yourself just enjoying your own company. Become your own best friend. If you see yourself as worthy of protection from harmful things it will be easier for you to enforce your boundaries.
If a person slips through those boundaries and you begin to feel your self esteem lowering, or warning signs appear, spend time with those that genuinely love you. Keep your friends close, maintain your relationships with them. Do the things that make you feel good about yourself until your self esteem is back up to the point you can create and enforce healthy boundaries to push that person out of your life.
If this person does not respect your boundaries go to someone in a position of authority to help you enforce them.
If you feel horrible about yourself, get outside help. Your self esteem may be too low for you to recognize that you don't deserve negative treatment. An outside viewpoint may be exactly what you
Remember you are worthy of love and respect. You are worthy of happiness.