The reason she's "okay" with firearms at home and in the car is that she has the the irrational feeling that these are inviolate "safe zones" where she believes you'll never need the gun anway.
Essentialy, she's okay with guns wherever she thinks she can be in condition white, because she thinks nothing will ever happen.
She's less secure on foot "out and about", however, this is where the emotional ill-logic continues. By the prior logic where she feels safe, she should want the gun near her where she feels the least safe. However, the opposite is true because she dosen't want you armed while out on foot as it makes her confront her feelings about being unsafe.
She feels so safe in the car and at home, that the presence and the need for self-defense dosen't shake her false world view of denial. When out on foot in public, her denial is on more shaky ground, and is more supceptible to being broken by the notion of going armed.
She is not afraid of you, your guns, or even of carry. She's terrified of being forced to acknowledge a world where you might need a gun.
All the advice about logic, getting her shooting so she know's firearms better and understands what they can and can't do, using Pax's excellent advice and materials is all very good….
However, in the meantime, you NEED to make it CLEAR in NO UNCERTAIN TERMS, that you WILL CARRY NOW, and AFTER you are married wherever it's legal for you to do so, and that it is NON-NEGOTIABLE, EVER. Because she's already emoting and not reasoning, you also need to make it clear that being married will not change things, and that having kids will not change things either.
And this conversation needs to happen as soon as possible.
So get the ground rules laid out before the "I DO's" and just maybe it will all work out. The way I finally got my wife to accept that I carry is I told her if the police could protect me and her from anything bad happening to us, then why was there so much crime in the city.
