Some people just don’t understand money. My sister is one of them. I just don’t understand how she can’t know not to spend money she doesn’t have!!
Luckily I don’t live with my sister. This isn’t the case with you and your girlfriend.
Money is serious business, and it’s just that – business. It helps to make that point in your discussions with her, as she can interpret your criticisms of her spending as being a criticism of herself (which it is, of course). Money ensures you pay bills on time and have security in terms of food and shelter.
The first step is to secure rent, utility, and food money. Establish a budget where she can see all of the information, she is to deposit half of rent and utilities and food costs (estimate for the first month) into a joint account used for paying these bills, and you do the same.
Then you are left with money for savings, investments, and spending on personal items. Here is where people should be free to make their own choices, but if you do intend to be a couple it’s good to establish savings and a plan for the future. So a certain amount should be put aside and invested or put into a high yield account. Then the rest is left to spend as you or her see fit.
Of course this is the plan for someone without any type of disability. Since she has a disability, this needs to be overcome. You can communicate to her just how important sorting all of this financial stuff out is to you and your future, and that without any improvement there is no future for the two of you. And then you can provide her with the name and number of a psychologist who can help.
Honestly, this is in my mind the best approach. It is simply not wise to marry a financially irresponsible person, because they will likely always be that way unless there’s some reason to change. Seeing their relationship end, or in trouble at least, because of their spending habits may be just the kick in the pants needed.