I can only speak from a personal perspective. It took me a long time to get therapy but I just kept beating down the doors (figuratively and once or twice pretty literally!). Keep going back to your GP, say that you are ready to accept help/therapy. Perhaps something like DBT which is tailored towards people with BPD? However, to take part in DBT you have to sign a contract to say you will do everything in your power to reduce (and eventually stop) self-harming with their help. Just keep trying. The moment you give up, they forget about you. If you keep going, they get bored of you eventually and give in.
LimaRoo