Your dad is wrong, you have a mental illness and it is BPD. This truly is a illness of the brain, it causes the amygdala of the brain to be overreactive, thus causing the rapid and intense emotions, and the pre-frontal cortex to be under active. This part of the brain is the reasoning center that for someone without BPD would allow them to rationalize their feelings. You are not a monster or any of the hundred negative things you call yourself either, you have BPD and one thing we do is hate ourselves. For some reason we give ourselves permission to say things to ourselves we would never let anyone else say to us, why we do that I'm not sure. From your writing it is obvious to me that you are a intelligent young man who has much to offer the world. This disease sucks to put it mildly, and I'm sorry that you have it and that it is causing you so much grief. As far as your question on where does the illness begin and you end, unfortunately you are looking for a black and white answer and there isn't one. You are always there and so is the illness in varying degrees. As far as your ex-friend forgiving you, I'm sure if she read this post she would. You are beating yourself up pretty good and should just try to give yourself a break.
I'm really sorry for the pain that you are going thru and I can relate to a lot of what you have written. I too have been there and done that, and since then moved on (even though it doesn't seem possible). If you haven't done DBT therapy I would STRONGLY recommend it for you, as I feel it would help greatly.
Please don't continue to beat yourself up over the past, you can't change it, only learn from it. Focus on the moment, it is the only thing you have control over. Don't worry about the future, it's not here yet. Live in the moment.