Just because you have BPD, doesn't mean that's the reason for the problems in your relationship. Since it is a long-term committed relationship, you should tell him. If he really loves you and is committed to you then he will accept you no matter what. Why hide secrets from him? That is one of the things that easily kill a relationship. How do you think he will feel finding out sooner than later. He might leave you for keeping the secret.

My fiance knows I have BPD. It didn't change our relationship. He just now has a label for my symptoms.

And it's not healthy to blame a diagnosis for your behavior. You can get better, but it takes work. Are you in therapy or on meds? That will show him that your invested in making the relationship healthier.