I've only recently been diagnosed with bpd and am currently on a waiting list for my local mental health unit. This means I know enough about bpd to recognize the symptoms I display, but I don't know enough to know when whatever I'm feeling is non-bpd related. 
My current relationship has seemed distant recently (on both ends),  I find myself getting easily irritated or frustrated with my girlfriend, and I have been thinking of breaking up because I don't feel any of the positives unless I'm with her in person and even then it's marginal. Issue being, is I know seeing only negatives can be part of splitting, so how do I know if I'm splitting or have genuinely lost interest in my current relationship?