Just saw this thread and thought I'd respond (have name changed as this isn't a condition that one would generally admit to openly).
I was diagnosed with BPD around the age of 20 I think, around a year after I had DD.
Essentially, BPD is a label for a combination of symptoms, which an individual may or may not have all of.
In my case, I can say that my BPD developed over my teenage years. I suffered depression and mood swings from 12 onwards, began self-harming at 16, engaging in risky sexual behaviour (threesomes, lesbian affairs, affairs with far far older married men etc). I felt wholly isolated from my peers at school, completely miserable and wanting some kind of emotional stability I think. I fell for completely inappropriate guys, couldn't maintain any kind of normal relationship and lurched from emotional crisis to emotional crisis. I did, however, manage to maintain straight A grades throughout my life so people didn't tend to focus on my issues since I was still achieving academically.
I had DD at 19 and for a year I managed to be completely focused on her and on motherhood. However, I began an affair with a woman I met at a mother and baby group (who was married), began self-harming again and took my first overdose a few days after DD's 1st birthday. For the following two years I was essentially totally fucked up.
I self-harmed most days, was an in-patient in a hospital twice (I had a choice - come in voluntarily or be sectioned) and took a ridiculously large overdose when DD was 2.
I couldn't cope with life at all. I had no strong sense of identity, continued to feel removed from my peers, felt that I didn't fit in anywhere and craved some kind of emotional balm that would soothe me, yet failed to find it.
When DD was 2 and I was an in-patient on the psych ward for the 2nd time, a nurse said something fairly profound to me which made me sit up and take stock of things. He told me I could choose to be a perpetual patient or I could choose to live properly. Whilst it's certainly easier said than done, that marked a turning point.
I haven't self-harmed in around 6 years now. I still think in a BPD way a lot of the time. I still get depressed and still have suicidal thoughts. However, I don't act on them. I've vowed that I'll never be hospitalised again. To all intents and purposes I'm living a fairly successful life. The notion of having a stable and happy relationship remains elusive to me but perhaps that will change as the years go on. I've had some therapy recently which has made me recognise patterns of behaviour which I previously hadn't acknowledged.
Anyway, apologies that this has turned into a mini-autobiography. I'm not sure if it's helped you. Probably Akhems' link is more useful