Partner does not understand my illness

My girlfriend and I have been together for almost 3 years. We live together and everything's fine - sort of.
She doesn't really get me, though. I have ADHD and Borderline Personality Disorder and she just doesn't get it. I know that it can be difficult to undetstand someone else's mental illness, but she's not even trying.
When I have a bad day where I can't control my emotions and everything's dark and crap she doesn't get that it's not the day for visit. Several times she's invited her father over for dinner or something else even though I have the worst day of the month. Of course she asks me if it's okay but how am I supposed to say "no you can't have your dad over!"
I get that I could, but I'm not that kind of person and honestly, shouldn't she be able to understand that this is not the day for visit? Or am I the wrong person here?

Have anyone else experienced episodes where a partner just doesn't get it? I'm really frustrated by this but it's not like I can make her understand if she doesn't want to.
I have tried telling her to google things about my illness and just read stuff about it to get a better knowledge but she never does.

For both me, and my husband were times when we didn't undertood each others struggles.

Mental ilnesses are really hard to understand from the perspective of healthy person or someone who looks at it as being not well developed character. It looks like excuse for bad behavior or laziness.

I think the important thing is to let her know and involve in the struggles you go through. Telling her how you feel deep down and how you are trying to work on it. She has to understand that you are prisoner of your ilness, not using it as an excuse to act the way you do.

You both have to talk and develop your own procedures when you are feeling down. You may show her some videos or articles that would help her understand you. The thing is that also she needs to know that you are trying to get better, that you just can't be well sometimes. When you are feeling down try to write down short talk that you will tell her every time so she feels thet you respect her like:

"I am really sorry but I am feeling very down today and I will be unable to treat you and your father right. I hope you understan that I have to just go through it today. I love you. Let's meet with your father another time, ok?"

It's important to comunicate and be respectful. I hope you both will find a solution to your problems.