I was not confident I could hide my pain... .
You did well in "taking the high road" and setting a boundary but the quote above, when you read it in a few years time and you are in a healthy relationship, will seem pathetic to you. When we look back at our stupid mistakes we see how foolish we were. What then makes healing harder, is forgiving ourselves for our stupidity because nobody else is responsible for that and we are the only ones who can put a stop to it. A pwBPD cannot feel about us the way we feel about them so our efforts to help them are in vain.
The fact that you are not confident is my concern. I would suggest that if you had confidence you wouldn't be in this position, correct? Sometimes we need to look within to see what makes us a lap dog rather than a tiger. You took the first step by setting a boundary here, but you should think about taking the final step and going "no contact" or you'll have more sleepless nights while she is being pleasured every night. That just doesn't sound fair to me.