The reality is that you can't really hide this sort of thing from someone who's close to you. If they care about you, you can only really explain as clearly as possible what the difficulties are and hope that they're supportive - I mean if the relationship continues they'd have to know eventually wouldn't they?

I think if you can get yourself into some sort of treatment at least it would help you in the more difficult times. If things get really bad you could try medication but again this would need to be given by a psychiatrist. I'm in the UK so I don't quite understand how it works where you are but if you go to ER could you at least be seen by someone then? They may be able to advise you. Other than that there are really only self help websites and local support groups.

Check out the BPD board if you haven't already as you mind find some of the advice there useful. Good luck.