Unfortunately, I cannot give you a diagnosis over the Internet. I would need to conduct a full and thorough evaluation as well as know more about your psychiatric history to be able to offer a reliable diagnosis. I will attempt to answer your questions based on the information that you have provided.
You mentioned BPD and bipolar disorder. I am not certain if these two are being used interchangeably (i.e. BPD= bipolar disorder) or if you are using the acronym BPD to mean borderline personality disorder. For purposes of my answer, I will assume that BPD refers to borderline personality disorder.
The mental health professionals who have offered you a diagnosis may be having difficulty because many of your symptoms, including trauma history, self-injury and suicidal thinking, are characteristic of multiple disorders such as borderline personality disorder, PTSD, bipolar disorder and schizophrenia. This may be why you are receiving different diagnoses from multiple mental health professionals. It is not uncommon to receive different diagnoses from different mental health professionals. Unfortunately, psychiatric diagnosis is not an exact science. I can understand your frustration.
As you mentioned, you only have some of the symptoms of borderline personality disorder. If this is true, then the conclusion would be that you have some of the traits of BPD but not enough to warrant the diagnosis.
One constant symptom that you have is command hallucinations. The voices are telling you to harm yourself. If it is the voices that are telling you to harm yourself then this would be a different type of self-injury than typically associated with borderline personality disorder. What is the difference? The motivation or the reason for the self-injury. In your case, it seems as if you are engaging in self-injury because you hear voices that are telling you to harm yourself. An individual with borderline personality disorder may engage in self-injury because he or she might believe that they “deserve” to suffer. The purpose in the latter instance is self-punishment. It does not seem as if you are deliberately harming yourself. The motivation or the reason for your self-harm is apparently not within your control. You seem to be engaging in self-injury because you are being “commanded” to and not because you are punishing yourself.
Command hallucinations are associated with psychotic disorders, specifically schizophrenia. It is not a symptom of borderline personality disorder. Individuals with borderline personality disorder can experience psychosis but it is rare and generally brief in duration. If you are engaging in self-harm because the voices are telling you to do so, this would not be a symptom of borderline personality disorder or PTSD. It would more likely be a symptom of schizophrenia or bipolar disorder.
To answer your specific question, symptoms of schizophrenia can lead one to engage in self-injury and to have suicidal thoughts. Individuals with schizophrenia may experience voices that “command” them to harm themselves. This is one example of how schizophrenia symptoms may lead to self-injury and suicidal thoughts.
I also want to clarify one aspect of your letter related to schizophrenia. You said “I don’t think I am violent enough to be schizophrenic.” Please understand that violence is not a symptom of schizophrenia. Unfortunately, there is an ongoing misconception that individuals with schizophrenia are violent. There are a small number of individuals with schizophrenia who are violent and it is these cases that tend to make media headlines. The facts are quite different from what is portrayed in the media. Individuals with schizophrenia are much more likely to be victims of violence rather than perpetrators of violence.
I suspect the diagnosis of schizophrenia may be the most accurate among those that you have presented but again, without extensive interviewing, it is too difficult for me to be certain.
I hope that my response answers your questions. Should you need clarification or have further questions please do not hesitate to write again. I wish you well. Please take care.