As a sufferer of organic brain disease and mast cell (and lots of other stuff) I have been in her shoes. In my case it took me telling my then other Hal’s “you are asking to pre-board the Titanic. Are you sure that’s what you REALLY want to do? Because the only thing worse than doing all this alone is to start down a path with someone and when I need them most they aren’t there.”. In my case she said yes, but her actions later proved otherwise.
There is guilt, doubt, and anxiety that if you truly open up to someone they will use it against you, abandon you, etc. I would guess she has been burned before by people close to her. This is the only way she knows how to cope with it. If you TRULY mean it when you say you know what you are in for then stay supportive and let her know you aren’t going anywhere. This may be a test that even she doesn’t realize she is doing to see if you really mean it when you say you’ll wait for her.
The other key is she needs appropriate medical treatment to control pain, mast cells, brain issues, and therapy to handle all this emotionally. Without therapy as a safe place for her to work thru some things this may never change. It is a SERIOUS CONSCIOUS effort to put ourselves in a relationship. Guilt and similar feelings are a huge burden/obstacle to overcome.