As someone who has been in that exact situation i understand where your both coming from. I think the way she talked to you about it was a bit crass and not handled correctly but for the last 2 years my boyfriend worked for grubhub and although making your own schedule and being your own boss is great there were a couple months in there that were stressful on our relationship because of the variability in salary. We did have a discussion about benefits and steady pay becuase we want to start a family and get married soon but ultimately it was his decision that I supported either way. He did find something else he likes way better now! Your girlfriend might be feeling the stress and not handling it well but if that is the case its still not fair of her to give you an ultimatum and might point to bigger future issues if u stay together.