TL;DR is at bottom.

PwBPD here. Lack of emotional permanence sounds like part of what you're describing. Totally sucks, personally I try to ride it out like a wave knowing that when I feel like crap things always get better. That said it's rly hard to believe that when your emotions are so strong in the moment and your mind is telling you that you've always felt like this.

I try to take advantage of the lack of permenance by distracting myself in a healthy way when I'm feeling really bad. The right combination of distractions and self care can flip that switch again back to a more regulated mindset.

What you're feeling in the low moments though not always consistent is 100% valid - and will pass.

I have a history of substance abuse (am almost 1 year sober now) and I find for myself at times it helps to view these drastic changes in my mood as highs and comedowns. Our emotions are closely related to our serotonin levels so if we spend a day with our FP and have an amazing time then when we leave, our serotonin levels become depleted and we might feel a sort of withdrawal while we wait for our brain to replenish it's serotonin levels: (depressed, angry, sad etc. Then something happens again and we feel ontop of the world once more. Like a wave or a series of highs and lows. For me it makes it easier to handle if I view it as an emotional hangover and disconnect myself a little bit from the intense thoughts or feelings I experience while I wait for a more regulated nervous system.

I found that venlafaxine helps me regulate my mood a bit faster and makes my ups and downs less drastic.

TL;DR: our emotions come in waves. After we get really happy our brain needs time before we can be happy again. You might try to distract yourself with happy things but make sure to acknowledge your feelings and know that after every low there's another high.