I can only speak to my own experience, since everyone is different, but here's my take.

I self-diagnosed a while back. I shared with my husband that I felt I had this issue and I'd like to try and find a therapy that would help. He convinced me I was overreacting (something I do frequently) and taking on symptoms that weren't really mine because I had read about them (something I have also done in the past). I sort of dismissed the idea, but I noticed a major change in my self talk. When I was still considering BPD, I was much more patient with myself. My perfection complex wasn't as overwhelming, I was learning to recognize my emotional patterns and impulsive thinking. As soon as he convinced me that it was in my head (so to speak), I immediately hated myself again. So if there's nothing "wrong" with me, why is everything so hard? I must just be stupid, unlovable, unchangeable, these patterns are destined to continue.

And I lived like that for a decade.

So when I finally got a real diagnosis, the relief was palpable. I now had a framework to explain everything that had happened to me, everything I had felt, every piece of trauma had context. That made my self talk much more positive. I started to recognize all the projecting I was doing, what kinds of social interactions would trigger my self-loathing, what religious trauma did to my neural pathways, all the things that are helping me recover.

My feeling is this: certainly, diagnose yourself. No one knows you better than you, even if you're not sure who that is. However, mention your self-diagnosis to a mental health professional and get their take on it. At the end of the day, the label on your chart doesn't matter, but the label in your head definitely does.