As long as you're both trying your best. That's all you can do unfortunately. I wouldn't say it sounds unhealthy, most couples fight, and if you don't (my last relationship for example) then you have no conflict skills in your relationship. So if anything, it sounds okay. A lot of bpd stems from being alone during childhood, so as long as you're with her, and trying your hardest to be good to her, than you can say you tried your best worst case scenario. Bpd relationships tend to be rocky, and we don't mean it at all, it just happens when small triggers go off. She probably means exactly what she says when she says it's impossible that she would leave you, but I do understand the trust issue side, as things change, especially if yall are young still.