The most important thing you said is that you haven’t seen a professional “in ages”. Diagnoses are the therapist’s best understanding at the time. Often as people’s treatment progresses or as a person matures, the diagnosis may shift. You really do need to go back to whoever diagnosed you in the first place to explain what has been happening. That person already knows a great deal of your history and can put it together with your present situation. Please follow through and make that appointment. If you are concerned that you may not be able to describe your experience well, just bring along your letter. You did a good job stating your concerns.
To answer your question: “How could I just black out and go on with my day?” Issues that can contribute to memory issues in young adults are depression, a sleep disorder, under-active thyroid and chronic stress. A side effect of some medications can also be the problem. Alcohol and substance abuse are also possible contributing factors. Since you have a history of borderline personality disorder, that you may inflate or deflate your experiences.
But do bear in mind that everyone has moments like this. You may be able to relate: Sometimes when we drive on a familiar route, we are surprised to see that we are further along than we thought we were. That does not indicate “blacking out”. Being very familiar with a routine lets a person put primary attention on something else while still safely carrying on the routine task. “Normal stuff” like doing your laundry is so routine that it allows your mind to think about other things.
Another possibility is that you dissociate more profoundly than you think you do. Sometimes this indicates Dissociative Identity Disorder where an “alter” personality who takes over. If you have a trauma history, this is something to explore. An indication that this might be the problem is your report that you have conversations you can’t remember. You need to talk to an experienced, licensed professional who specializes in trauma to decide if this might be true for you.
Without knowing you, I can only offer the list of possibilities. Please go back to your therapist for a complete evaluation.
You wrote to us because whatever is going on is troubling to you. I think you should take you instincts seriously. You know something isn’t right. See your therapist as soon as possible.
I wish you well.