This sounds so hard, and it is so difficult to feel this way. I feel this way often with my friends, especially when I don't get the quick responses I like to get. It makes me feel abandoned. I think if you really value this friendship, which it sounds like you really do, you should open up to your friend and tell him your concerns about BPD and how you're feeling. It sounds like he would be totally supportive and willing to help you, maybe even more so if he has an idea what is going on.