When he told me he has bpd, he runs when falls in love, I thought I could save him

How naive I was, I stayed because I thought I would be the exception, because I thought if he loves me enough he then would change. I bought books for him, went to see him when he was crazy. I offered my insurance when he was not well mentally. At least, I thought he would try to be better for me. Ironically, it’s the opposite. He ran faster, abandoned me cruelly. Now, he is with someone else, stayed with them, happily. Me, still stuck here. What have I done to deserve this?