It's a long road because it's not as simple to be diagnosed as for example with physical illnesses. Some people have been through more than a dozen diagnoses before one is reached with medication/support that is helpful. Sometimes it depends on the "in" diagnosis for the moment and how you are at a single appointment.
Some will come with prejudice and discrimination within the mental health system and by their nature won't be removed. Some can exist with other conditions. Some professionals will only diagnose someone who self harms with borderline personality disorder, likewise suicide attempts even though they can exist as part of other conditions e.g. PTSD.
I've never found labels helpful, for no other reason than the prejudice I faced because of them. Personally I concentrated on what I found helped me. And by that I mean helped me. I was for example told under no circumstances to see support from the hearing voices network (they may or not still be around with their support groups) because I have a diagnosis of "borderline" which I was told means I'm just attention seeking (this was from a professional and is NOT my opinion) and I do not agree with it anyway - the diagnosis having been made by a questionnaire which was heavily edited to only include borderline traits and those that weren't were rewritten to meet the criteria. Consequently treatment was useless. Anyway, hearing voices network was about coping with voices (I was told mine are just me being a bit silly) I lied about my diagnosis to them (they wouldn't accept "borderlines") and went along to see if anything would help - fully aware it could make things worse, some things worked for me and I found the group helpful.
I cannot medically have meds due to neurological issues which the mental health team dismiss on the grounds of me being "borderline"; it's all rather rediculous to be honest. But here you are given that diagnosis first and must prove you are not.
Over the years of being allowed access to services (long story as to why it stopped, and no I didn't get better or do anything dreadful to staff or become "too reliant") I was diagnosed with the following :
Borderline
Severe depression that required hospitalisation multiple times
Anxiety
Life threatening medical phobia
Schizoid personality disorder (sort of bipolar and schizophrenia combined)
Schizophrenia
Bipolar
PTSD
Complex PTSD
Those are the main ones, there are others too but it's almost 1am and I need to try and get some rest. The last two I was told are just alternative names for borderline. Many were dismissed because I have a degree (yes really).
Over the years treatments included things like being advised to speak to a medium to find out who the voices were - I was advised by a mental health nurse to do that; they thought it was a friend who and ended their life and I was responsible for them doing it (I was told it was my fault). Pay for private assessment, again I've a degree and have worked so was told to get loans and credit cards and use the debt to encourage me to get better quicker (in the end their diagnoses were binned because they were private not NHS). NHS therapy was terminated after I was told I must have suffered severe abuse because of borderline label, the abuse I had gone through was dismissed as I was over 6 and so my long dead foster parents who I adored were blamed and I was repeatedly told it MUST have happened because in the 70s babies were given to anyone and no one would want someone else's unwanted kid except to abuse them and I ended up after weeks of hearing this losing my temper and then being told that meant they were right. I refused to go back. What I needed help with coping with was dismissed by this person as they were obsessed with foster carers being abusers (I wasn't alone being told this).
Hence, it can be a long road; with many cul de sacs and, can in itself be traumatising.