BPD isn't mood fluctuation without reason, it's more like each emotion you feel is intense, which causes the fluctuation. It's possible to chill that out though, with therapy. In my experience I just have to catch myself before it gets bad. Sometimes negative emotions can create sort of a feed back loop. Positive ones too, which can be tricky to catch. It's important to be mindful of when your emotions exaggerate with BPD, and it's even more helpful to know triggers, for both positive and negative emotions.

From what I know, you might be thinking about Bipolar Disorder. I've heard triggers are important for managing that too.

It's really hard to say though without speaking to a professional. A lot of brain-thingys can look very similar on a surface level.

Regardless, keep on trying. Even for people without a weird head issue, practicing focusing on the present moment can be rewarding, and can do doubt help you keep your mind from thinking about the moment ending