This sounds like she's been surrounded by Enablers since her very childhood, because of which she either doesn't want to get better or doesn't believe she can. Being mad and frustrated at her, while not completely unjustified, won't go far in solving the issue, and I daresay this is something that should've been talked about at the very beginning of the relationship. Anyways, it takes a lot of patience and love to coax someone out of their shell of shame, and very small incidents can scurry them back inside. How severe is her stutter? Could you simulate some situations (Like You calling her over the phone, or gamifying a day at the shop) so she can build some confidence? Lots of people have suggested therapy, that can be an avenue, provided she's willing to do it. Most importantly, see to it that the small wins in her life are remembered and celebrated. Only that is the way through.