So, your GF is 26 yrs old. She has no job and is freeloading, living with you, and emotionally blackmailing you into marriage. She decides cleaning the house and cooking is a wife's job, and since she doesn't hold that title, she's going to make things more uncomfortable until you marry her. It doesn't bother her that she is not contributing financially, and now she's not helping with chores. She's just going to lay around all day, waiting to have the baby. Lovely young woman you have there! Consider this as a preview of your future with her.
Wolfie, just wanted to reiterate this. I'm not sure I've stated it as succinctly as Sandi did but I also really get the sense that your GF is manipulating you and may have narcissistic tendencies. I suspect she's gas-lighting you which is probably why you feel so much guilt. It may very well all be part of her plan. I can absolutely guarantee you that if you do marry her, she will do even LESS around the house, not more. There will be LESS sex, not more. What will there be more of? Lies, gaslighting and manipulation.
I could be wrong, and May could be right that she's just young and confused. But I still firmly believe that either way, the best thing right now for you, for her and for your future relationship is to move her out. It's fine to keep going to therapy and seeing her if both of you want that, but you really need to be separate to properly work things out and decide what you want.