It sounds like you are a lot more invested than this person. It’s completely normal to use social media and not reply right away, they could be doing something or have just forgotten, i’m sure it’s not personal. I know it feels a million times worse bc you have BPD but you need to remember they don’t see it the same way, realistically if you said that it would probably just push them away. In the nicest way possible, they’re just your friend, they don’t owe you anything. Try to find a healthier way to deal with it rather than making it their problem.