New diagnoses can be terrifying, especially when you don't see them coming. To me for a while it feels like someone has shaken the world and left it diagonal. I feel horrified at myself and what this means about me. I'm so sorry that this caught you off guard. Hearing that it came from an old therapist would make me feel hurt and vulnerable.

A few thoughts, echoing much of what others have already said but that I agree with.

Many mental health practitioners feel like diagnoses aren't very important and treatment is. My current therapist wrote on my chart when we first met that I had depression. A couple years later, we realized I actually have bipolar. Just a month or so ago, I found out that his office was still billing my visits with a depression code. I was really hurt and asked him, "Do you not believe I have bipolar? Are you just pretending, going along with something I want to believe about myself?"
He said, "Oh, I never thought to change the diagnosis code. I usually just write a code initially so I can meet with people, and then I don't really think about it again. I just focus on what we're working at any given time."

Your therapist may have diagnosed you with BPD or something else but just never mentioned it because she didn't think about using those exact words. She preferred to use the words like "binge drinking."

Even with that said, I would be very upset if I left therapy with my current therapist and found in a letter that he had given me other diagnoses but never actually told me. I'd feel betrayed. I'd feel like he didn't trust me, like he went over my head. If you're having those feelings, I think they're totally understandable.

Could you talk to her about it?

If he had done that and then I couldn't talk to him, I'd probably be trying to reframe it in my head like I've done above. I'd list reasons like, "He may have forgotten to tell me. He may have cared more about how I was doing than about giving me a label." That would reduce my distress.

One reason you may have not been referred to a psychiatrist is that if you really do have BPD, that disorder is most effectively treated with therapy, then meds (in contrast to bipolar, for instance, which has to be treated by meds first and most importantly, and then therapy can help). Therefore, they may have been treating with therapy and not meds.

Your psychiatrist said that you should be on higher doses. Your GP probably has no idea. GP have almost no training in psychiatry. They're better for treating simple depression or anxiety, not the multiple co-morbid disorders or really complex disorders that many of us here on this forum have.

Last - I'm not sure where you're getting the idea that psychologists don't diagnose? I think TimtheEnchanter was talking from the perspective of having providers who didn't diagnose well, but they did diagnose. Therapists and psychologists are THE diagnosers. They diagnose almost all mental illness, with a small number of people originally diagnosed by a psychiatrist. (here in America, that is)

I really hope you're able to find some peace. This is a really hard spot to be in.