To be honest, unless someone has a close family member or has experienced a serious illness, most people have no idea. They don't mean to come across as unsympathetic or unkind, they are simply ignorant, with a complete lack of understanding.
I think this is likely the case for your friend. Of course she wants you to come to her hen do, but she doesn't realise that her messages are putting you under pressure, and likely making you feel bad/guilty.
It's very very difficult for someone to understand a serious or chronic illness, and like I said above, unless you've walked in those shoes, you can't.
Tell your friend that you probably won't know until the day, if you are likely to be able to come or not. She might find it disappointing, if you can't, which you understand. However, her disappointment is nothing compared to the frustration you feel towards your own body, because you are living it, and to be grateful she isn't.
I have had a brain injury due to contracting bacterial meningitis and septicemia, it has caused a lot of side effects that aren't obvious when you look at me. I forget things very easily, due to memory issues, and it can be frustrating for others. They aren't living it though are they, they don't get embarrassed or sometimes humiliated, because people think they're a bit dim! My brain works slower, because it takes longer to process information but I'm definitely not stupid! I don't expect people to understand, how can they? They've not experienced the same and it's difficult for anyone who hasn't to comprehend.