Opinions will vary from psychiatrist to psychiatrist. No diagnosis should be an opinion but when it comes to mental illness, it usually is - Psychiatry and Psychology are not exact sciences and are more fields of educated guesses, mostly due to the lack of physical evidence. With physcial conditions (such as arthritis, hypertension, cardiac arrhythmia, cancer, ventricular fibrillation etc) there is clear diagnostic evidence that can be obtained through simple tests (blood tests, ECGs, MRI & CT scans, lumbar punctures etc). Mental illness however are diagnosed according to behavioural patterns and due to the vast amount of mental illnesses and conditions, symptoms often overlap.

The best way to get a more accurate diagnosis is to get a second opinion - one from two different psychiatrists. If two psychiatrists say the same thing, you can be more sure of the diagnoses. If the opinions differ, you either get a third opinion (which is often difficult) or you can ask one or both psychiatrists to reconsider. If two psychiatrists say one diagnoses, and a third says something different, you can go for the majority.

BPD and Bipolar are the two conditions which are most often misdiagnosed because they are so similar and it can be difficult for a psychiatrist to conclusively differentiate between the two. I have reassurance of my Bipolar Diagnoses because my behaviours were recorded over the course of four years by the mental health team before I was diagnosed, and my disorder wasn't diagnosed until I'd spent two months in the same psychiatric unit. Many do not have this certainty and faith in their diagnosis.

It is not too uncommon for BPD and Bipolar Disorder to coexist either. You can have both and my psychiatrist has her suspicions that that may be the case for me, so you wouldn't be the first person on these forums with both conditions (I am also not sure about it being both so myself and my psychiatrist are on the same level there) but there are others.

Now, it may be that you havent been misdiagnosed and you do have BPD. We cannot say. What we do know is that the highs experienced by those with BPD can be similar to hypomania in those with Bipolar Disorder, even if that's to a lower extent. With BPD, the mood swings are more common than a typical diagnosis of Bipolar Disorder in which moods tend to change after a number of months (episodic change in states of hypomania, mania, typical or depression). There are rapid cycling diagnostic criteria for Bipolar Disorder in which moods change more frequently with no sustained period of stability inbetween but this is rarer. As for ultra rapid cycling, this is a very rare occurence and there aren't actually any diagnostic criteria for it - it is not included in the DSM. Hypomania also lasts a minimum of four days, typically.

It is important to remember that rapid cycling Bipolar is not a diagnosis but is actually a "course identifier".

I'd say to push your point of your doubt over your diagnosis and to keep a mood diary for a minimum of three months to show to a psychiatrist and also assist in your treatment.

I hope that helps you to get some clarity. Keep us updated.