The main difference between these two diagnoses is that in order to be diagnosed with bipolar, you must experience both depression and mania/hypomania. Depression is only part of bipolar, but in order to be diagnosed, you have to have both. Bipolar cycles, whereas depression doesn’t. Depression doesn’t mean you don’t have good days or that you are down all the time, but that’s different than mania.
If you feel you may be misdiagnosed, then I would encourage you to talk to your psychiatrist. Even if it’s not an issue of misdiagnosis, it may be an issue of needing different meds.
Also just wanted to note that bpd is borderline personality disorder and not bipolar.