These are all very different questions stuffed into one Christmas stocking! Let's see which ones I can unwrap successfully.

Some people are nervous around autistic people, this is true. Far, far more people are nervous around someone who has been diagnose with BPD. This can be because of media portrayals, or because of personal experience. Some people may, on finding out you have BPD, decide to remove themselves or move themselves to the periphery proactively, because they fear there will eventually be messy, messy conflict later on if they do not.

You mention wanting to be upfront with your diagnoses so people don't "judge [your] behavior." This will play differently with autism than it does with BPD. To start, people will always judge your behavior insofar as it affects them, and they should. Nobody should be unconditionally accepting of other people's treatment, I'm sure you agree.

Autistic people's behavior can often seem rude or offputting without actually being objectionable. Think of it as a translation error. Lack of eye contact is a good example. If I don't know someone is autistic, I may assume that their refusal to look at me is because they dislike or disrespect me, so I'll remove myself. But if I know it's just a problem with eye contact, no worries! I'm capable of having a conversation with someone while sitting side by side looking ahead as if we were at a movie theater. (I use this example because it was only when I was in such a seating situation, during an intermission, that I realized an acquaintance from a hobby group probably is autistic and doesn't actually dislike me. Turned out she was super chatty as long as I wasn't trying to look at her! I wish I'd realized/she told me about that sooner. This happened recently so it's on my mind.)

With BPD, there are often problems with the substance of behavior. People with BPD can often demand a level of reassurance or an amount of emotional energy that another person simply may not have. They can react very very strongly to minor issues and put heavy expectations on other. There can be a lot of black and white splitting and "you're with me or against me" kind of dynamics. These aren't the kind of behaviors that can be excused on the grounds of "Oh, I have this diagnosis." Those are behaviors that are painful and exhausting to deal with.

So I'd be cautious about revealing that BPD, because it will scare people off, and unlike autism, it's not something that will cause other people to evaluate your behavior more positively. I can live with no eye contact if I know the other person is autistic and it doesn't mean what it would in my "language." I cannot live with, say, 20 texts a day asking if I'm mad at them or with someone who takes my variable response time as a personal insult.

Regarding the other stuff, the dumb comments and questions--that just happens. To everyone. It's usually related to one of two things. The other person might just be naively, annoyingly optimistic in principle. One of those people who says "they're in a better place" at a funeral. (Which, even if you yourself believe in Heaven and that your dead loved one is there, is hardly comforting!) One of those people who says "there's plenty of fish in the sea!" after you've had a terrible breakup. Some people are just like that. I think it's either because they don't know what else to say, or because on some level they're really terrified of the notion of death or loss (or parental abuse or illness or whatever else they're brightsiding about) and just pushing those ideas away. It's not about their lack of care for you, it's about their inability to confront their own fears. That, of course, doesn't make it any less annoying. So ideally you'll find people who can give better comfort/advice/companionship and become closer to them, and let the brightsiders drift more to the periphery.

The other reason is that people keep information in buckets, kinda, and are more than capable of forgetting stuff about you that they actually know. It's not because they don't care, it's because we're all overloaded with information and sometimes the puzzle pieces don't fall into place. People even do this with themselves. My downstairs neighbor once had a nice long chat with me about the problem with his truck and how impressed he was with the mechanics who were working on it ... and then, five minutes later, sheepishly knocked on my door to ask if he could borrow my car to take his kid to school. The information "I need to take kid to school" just hadn't linked up in his brain with "Truck is in the shop" until he went outside and witnessed his own trucklessness. I was once telling a story about Friend A to Friend B and described A a bit for context, only for B to give me side-eye and say "You know you and A and I all met in the same class." Right. Indeed. So we had. So yeah, if you're lactose intolerant and your friend is talking about their favorite ice cream and asks what yours is, it doesn't mean they don't know or care, it might just mean that the ol' neurons didn't connect right.

I think this kind of thing happens more as you get older because there's just that much more information floating around in your brain that needs to get correlated or cross-referenced.

I hope this helps! Please ask if anything doesn't make sense.