Disclaimer: AGH so the last time I was inpatient, my therapist says to me that I'm showing a lot of symptoms of bpd and we should talk about it. I agree, bring it up multiple times, and get discharged before we ever actually do, so I'm not officially diagnosed. I also deal with imposter syndrome, so basically always questioning whether I *really* experience these symptoms or whether I *really* have major Depressive disorder, so the short version is:
I experience all the symptoms, as told by multiple friends AND therapists, but since I was never diagnosed I feel that I can't comment without expressly pointing out the context.
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So anyway, as far as dealing with symptoms, I've found that what works best for me is journaling and (highly uncomfortable) conversation. For example, if I'm starting to obsess over the fact that my partner might only be telling me that they like my conversations but really find me annoying and boring as shit... I confront that.
If I'm having trouble verbalizing, I write that shit down in a letter and text them about the fact that I have it before I can chicken out. Usually, the anxiety of waiting for them to read overpowers and I end up word vomiting my worries all over the place. I tell them bluntly that, "I worry you actually find me annoying and uninteresting, and I know it's probably not true, but I can't help but obsess and it's making me feel really insecure." I might then explain my rationalizations.
If I can't talk to other people, that's where the Journaling REALLY plays a role. I know when I'm in a state/heavily symptomatic, I'm not in a logical headspace. Logging my experiences and feelings can help me map out what's going on and work on the patterns and traps that I fall into. It helps me feel productive about my issues and enables me to problem solve more effectively.
Above all, I try to remember that I won't always be successful, but that I DO learn from my mistakes, even when it feels like I'm cycling. Acknowledging it is the first step, and taking action, however small, is extremely difficult and very important at any size.
I try to apply this to other areas (when I'm depressed, anxious, etc), and I find that communication and writing are great for feeling less stuck.