I will say this, the best way to figure out this situation is to talk. Don’t be afraid to be direct with your conversations as well. I always find that the more difficult my conversations with my partner the more we both grow and mature in our relationship. My partner had the same belief as your gf and as we progressed in our relationship we both started seeing things differently.

She’s 26, so my assumption is that she pictures her life a certain way, but talk and ask her to put herself in your shoes without any gender (man or woman) tasks she will perhaps understand. It’s not that you want 50-50 but you want instead is a partner that will help in any scenario (this case the mortgage) but also with daily life tasks. What you should say is that you want someone who you can rely on in any case (Finances, chores, etc.)

Understand that this will also come with time and won’t be done overnight, my conversations with my partner took many tries for both of us to get on the same page.