By trying to squash your self esteem the controlling personality hopes to prevent you from ever leaving or questioning them. They want you to think ‘I can’t leave because I’d never find anyone else’. At the same time they want to bring your self esteem down so that you don’t trust your own opinion and so that you turn to your partner instead for advice. This might be as simple as them avoiding giving you compliments, or as blatant as them actually saying ‘no one else would find you attractive’. Regardless of whether the motives here are that the person is controlling, if anyone tries to take your confidence away from you then this is a bad and destructive relationship and you should end it.