BPD best friend - advice?

Hi everyone,

I'm new here and looking for advice. My best friend (diagnosed with severe depression) hasn't been officially diagnosed, but seems to fit BPD criteria too. His family don't know about either the depression or BPD-like symptoms, they are totally out of the picture. He's alienated all of his other friends (or in his words, they've abandoned him) and as a result I am his friend, carer and parent all in one. He has threatened to take his life if I tell anyone, even a mental health professional or a friend, about what has been going on (and he means it).

My friend is terrified of being alone, constantly blames me for 'leaving' or 'abandoning' (even if that means sitting across the table instead of beside him, meeting other people, or having to leave somewhere early). He also struggles with very low self-esteem / self-hatred and anger. He is very sensitive to things I say and construes them as hurtful / cruel even when I say them as kindly and caringly as possible. We spend more and more time together, which is taking me away from my own family, and yet he feels more and more alone. He also feels 'lost', does not know who he is. He is barely able to get through each day - although this may be more due to the depression.

I'm really tired, stressed and sad. I don't know if BPD is the right diagnosis. Over the past year or so I've just been giving in more and more, placing his needs first, and there are almost no boundaries left in our friendship... I'm just so afraid of antagonising him by saying no to anything. I'd really appreciate any advice as to what to do! This seems like a really great group.