my (23f) girlfriend (19f) shuts down when she feels bad. how can i help her??

i am autistic and due to this i feel a huge amount of empathy and whenever she is off or sad i can tell straight away and it affects my mood too. she has PTSD and she basically just shuts down and any semblance of herself disappears. she basically just becomes a shell of herself, she starts small talking me and acting like she barely knows me. when this is not happening, we have such an amazing relationship. i’m crazy in love with her and i’d love us to be able to be together forever and we’ve both spoken about how we would love this.

however, i just don’t know how to cope when she does this. even though she reassures me that she’s not off because of me, i can’t help but feel like i’ve done something wrong. i just don’t know how to help in these situations as she says that if she tells me what’s wrong it ‘becomes real’. we then end up in a situation where we’re both upset and we just kind of get frustrated with each other. the solution could be to just not see each other when one of us feels bad (the same happens when i’m upset, she just shuts down) but obviously that’s not healthy or sustainable. i’m just not sure where to go because neither of us really know what to do. i know that relationships can’t be all good but it’s affecting my mental health and i just want to be able to help her. does anyone have any advice??