Both borderline personality disorder and bipolar are characterized by severe mood swings. These are some of the differences between BPD and bipolar mood symptoms.
One of the biggest differences is the typical time of the mood swings. Bipolar moods (manias vs. depression) tend to last for days up to months. Some can be shorter, which is unusual, and some can be longer, which is also unusual. Typically, BPD moods last from minutes to days. Obviously, there is some overlap, where the differences come down to other symptoms experienced.
In my experience, bipolar generally experiences manias or hypomanias, where the feeling is a sort of “high”. Borderline “high”s are usually more aligned with irritability rather than mania. I’d guess that these moods are interchangeable when it comes down to it, but my hypomanias related to my bipolar are different from the irritability related to the BPD. I lash out due to this irritability, which can sometimes occur in hypomania, but usually the hypomania is more feeling like I’m on top of the world rather than lashing out at others. This may not be the case with everyone though. There is a lot of overlap, as I’ve said, and not everyone has the same experiences. Usually, if an emotional high can be felt, it can manifest as irritability as well as happiness, simply due to the energy in the mood. Even with depression, younger people especially can express this as irritability rather than the usual sadness.
BPD moods usually have triggers, whether they’re big or small. Bipolar mood swings can occur with no trigger. For example, a minor disappointment in BPD can cause a severe depression or anger. Sometimes, if it manifests as depression, it evolves into anger instead, as the mind’s self-defense mechanism of sorts. A positive trigger can bring that mood back up. BPD mood swings are almost like a child’s emotional state, where fits can be thrown over small things and then a small satisfaction erases that feeling entirely. Bipolar mood swings are harder to explain, as they are usually more controlled simply by the chemicals in the brain and neurotransmitter burnout.
If it’s difficult to distinguish BPD from bipolar due to mood instability similarities, the other symptoms experienced are what a diagnosis comes down to. In past episodes, I’ve discussed symptoms of BPD, which include a variety of things such as self-esteem issues, self-harm issues, identity issues, relationship issues, etc.. I believe these don’t include psychotic symptoms such as hallucinations or delusions, which can be included in bipolar mania.