Your decrease in intense emotions may be a result of you simply growing older. Mental healthcare professionals typically avoid diagnosing anyone with a personality disorder until they are at least 18 because being a teenager can often mimic BPD.

I'm not saying you don't have it because obviously I'm no doctor and I don't know you personally but it's possible you also don't.

Assuming that you do have BPD, it's definitely not uncommon for your symptoms to sort of "settle down" when you aren't romantically involved with someone, since relationships tend to bring out the worst aspects of the disorder.

Again, I'm not a doctor but I can compare some experiences I had at your age. I'm 22 now and I was diagnosed with BPD when I was 20.

At age 17, I was nowhere near the "passenger" you're describing yourself to be. I was a hurricane trapped in a tiny human body.

As years went on my turbulence never faded. That's why they were so confident in my diagnosis, because it has persisted into adulthood rather than dying down as a teen like it does for most people who have BPD traits.

I'm still turbulent but I have considerably more control over my reactions to things after a long time of therapy and DBT (though I could use more work).

That's just me, though. No one is the same, even fellow BPD sufferers. I would suggest seeing a psychiatrist and telling them exactly what you told us here.

(Also I self harmed because of a video game too once. But it was because I was frustrated at the game itself. Scary how quickly it can go from 0 to 100)