This is difficult territory. There is no way to tell. All you know is what they tell you, and if you have a doubtful nature, it won't really matter if they're telling the truth anyway. (I can certainly vouch for this firsthand.) How do you feel about the relationship? Is it worth persisting? Are you in love? Is it worth working out? It can be difficult trying to work out core relationship issues when you are, at the same time, working on serious mental health stuff, especially something like Borderline that targets relationships. Are you getting the help and support you need to deal with any of this?