She didn't 'steal' your boyfriend. He decided to leave you and get into a relationship with her. If you want to blame anyone, blame him as the person who actually betrayed your trust.
Or, alternatively, just move on. This sort of attitude really won't bring you any sort of closure from this. If he has betrayed your trust, he is not worth being with. If he hasn't and has just chosen not to be with you, it was also never realistic for him to be with you long term. Either way, let him go, move on with your life, and find someone who actually wants to be in a relationship with you.