My sister is only 18 but suffers from what seems to be anxiety in social contexts, self esteem issues and trust issues with her boyfriend. She has been in a couple bad relationships in the past, where they have been controlling, have cheated on her and emotionally abused her. She is now in a relationship with an older boy who I believe to be really good for her, more mature and cares for her. However she is haunted by the ghosts of past relationships and cannot trust her new boyfriend. She is constantly panicking that he will want to leave her, gets jealous, or stressed out if he doesn't reply to her for a short while. She has recently started a new full time job; i had hoped that this would help her mature, become engrossed in something where she wouldn't be checking up on her boyfriend all day and she would grow her confidence. She is an absolutely stunning yet constantly puts herself down and compares herself to her boyfriend's ex-girlfriends, asking why on earth would he be with her other them?! It's gotten to the point where she always gets upset with him over something and is pushing him away as he is constantly being questioned. She really cares for him and is terrified of losing him. She seems to react over small events that she thinks threatens her, and other times lets it build up so that she explodes at him. I fear he will be pushed to his limit and will have to leave her for his own happiness. Her insecurity in her relationship is also seen socially; she struggles speaking to people in restaurants, shops, banks etc, on the phone, was so nervous on a train it made her physically sick.
Sometimes I think she is attention seeking and frankly being stupid, other time I fear that it may be something deeper and she genuinely needs help to address her self doubts. I do, too, also fear that she relies on relationships for her own happiness. She has hardly been single since the age of 16 and I think she hasn't learned to build her own happiness.
Any advice on how to help her outlook and attitude towards relationships and herself, please?