advice about my boyfriend who has BPD

Hi all

I have been with my partner for three years and four months ago he asked me to marry him. I love him dearly and have been very understanding of his problems. I believe he is a good person but I do get exhausted by some of his issues regarding guilt and shame. He has a mild form of the disorder, according to his therapist, no drug or drink problems, he is not irresponsible with money and does not self harm. There is a problem though which keeps rearing its ugly head about every four or five months. he becomes very depressed/anxious and struggles to believe he is worthwhile or has a future, he feels empty and ashamed. I have learned that this hits a crisis and then he seems to get better, but I know he suffers and he pushes me away. Last week he hit this wall again and told me he couldnt ever offer me a proper relationship and that i deserved better. he never devalues me in fact quite the opposite he sees me as perfect and someone he has no chance of keeping. I find this impossible as if I try to reason with him says my words mean nothing and he knows how he feels.
I have a demanding job as a support worker with homeless people, I know how to be supportive and practical but I am getting tired of all his uncertainty and starting to feel I am making a mistake. When he is like this he is difficult and hurtful and I wonder why I stay but I refuse to call him a monster like he considers himself.
He is getting therapy but not DBT. I have told him to ask his therapist for this and he says he will when he sees him tomorrow. Can I ask people here if DBT works and if there are other therapies that help BPD people get better. I am not going to abandon him without a fight but I am starting to wonder if he can ever be happy and what the impact will be long term if he doesnt get some relief from these problems.

thanks everybody