I've read that ASD and BPD diagnoses are often mixed up, as on the surface they seem to have a lot in common. Apparently females with ASD may be given a diagnosis of BPD because they're seen as predominantly male and female disorders respectively. While in theory it's possible to have both, ASD symptoms of course become apparent early in life, while BPD symptoms typically appear in adolescence.
I was being assessed for BPD when I read about all this. When I pointed out that my cousins have ASD and my Mum, Uncle and Grandad could probably also be diagnosed, they didn't continue the BPD assessment and put me on the ASD assessment waiting list instead.
As for medication...it's a personal choice, but in case it helps I'll share my story. I mostly regret taking SSRIs because they make me feel/behave "less autistic", and I can't stop them due to withdrawal symptoms as soon as I start tapering. On the other hand, they do help me sleep and reduce my IBS symptoms, and I doubt I'd have finished my PhD without them (not that having a PhD has done me any good). I've not found anything else I tolerate, other than diazepam, and I think intolerance to meds is quite common in ASD.
As for DBT, I've read all the self help stuff and there's some useful stuff if you've not already discovered it just through trying to cope generally. Apparently the group therapy can be difficult for quieter types due to the nature of DBT. Still, I can't see any harm in trying.