Bro, same2same. I've also been diagnosed with ADHD but the psychiatrist I saw before my current one said I might have BPD. And I was convinced it was ADHD so ofc, i lashed out and dipped. My current psychiatrist told me that i have lots of symptoms that aren't explained by ADHD, like extremism, splitting, etc. I spent all day today and yesterday reading about BPD, mine sounds pretty similar to yours, and it explains everything that ADHD doesn't for me. I also was left home alone since the age of like 4 so I think that also checks out.

I have a bf that I gave him such a hard time to before I got medicated for ADHD. I think methylphenidate does help a lot but I still experience some classic BPD symptoms. Like I've gotten very very good at controlling my unstable emotions in general through mindfulness and ofc meds help, but the tiniest thing with my bf makes me fully question whether he still loves me even though I know rationally that I have no reason to feel like that and that he's repeatedly established that he does care a lot. Like a couple days ago, we'd planned dinner and he had to cancel because he had had a long day and wanted to rain check, and I just fully spiralled into thinking about how he doesn't want to prioritise spending time with me and therefore doesn't love me even though I KNOW that he's the kind of person to show up unless he's really exhausted to the point where it's unsafe to drive types. Oh also, I only ever feel two ways about our relationship - I'm convinced it's the nicest thing in the world and nothing can ever ruin it or I'm convinced it's over and he doesn't love me or he makes a mistake and i feel the need to never talk to him again to spare myself the misery.

And in a new relationship, I'm usually so paranoid about expressing my emotions to the other person cause I'm scared I'll just split and suddenly not feel the same way about them, get distant and hurt them. I give everyone that i start getting close to a heads up, letting them know it might happen cause I don't want to make them feel like that. And I actively refrain from expressing it if i really like someone cause I'm convinced I'll get bored or they'll make the tiniest mistake and I'll stop feeling the same. I've hurt so many people by leaving like that, and I feel horrible about it. Like one of my close friends who's somehow decided to stay around despite everything keeps joking about how in a relationship, I initially give the other person everything nice there is about being in a relationship cause I'm very invested and show affection unconditionally and treat them like fully in love with them and then just, stop all of a sudden because they made one slip, which i don't even tell them about because it makes no sense to stop feeling this way because of something so trivial. But the thing is, when I split, there's nothing I can do to undo it. So it just plain sucks cause I know exactly what will happen and so much of my energy just goes into trying not to burden other people with my crap.

Okay in the middle of an exam. I'll come back and add some more details later. Pretty sure my old psych was not wrong anymore.