I don't trust my girlfriend anymore after lying to me and conveniently losing her memories
Umm can you explain what you are saying?
It is not true that 'the dog that bites you always remembers you.'
It could definitely be really hard to trust someone after you catch them in a lie. However, it can be pretty rude to hold a grudge against someone if they have an actual medical problem, you should seek medical help for her to see if her condition is true or not. Automatically assuming that she is faking her condition could cause issues with the relationship down the line.
I know what to do, I just don't want to think about this anymore.
Well if it is a small lie I don't think it really matters that much. Maybe you can ask her why she would lie it could be for a silly reason. I think both of you could really do better with help to communicate with each other.