Mick, there is a strange phenomenon that alzheimer patients often go
through.  They sort of live their life backwards, and all sorts of memories
from their past will come to light.   She could be experiencing a delusion
that you are her boyfriend or her husband or something like that.   No
amount of reminding her who you are really is going to work.   She can't
remember what you tell her anyway.

You have to remember that her brain cells are dying off more every day, and
she is not at all rational anymore.   You will need to get her doctor to be
aware of this behavior and perhaps some medication might be in order if she
is getting violent.   You need to realize that this is an illness that makes
the person experience all sorts of things, and delusions can be horrible for
her as well as for you.   It is a good thing we live in the times we do when
there ARE medications that can help.    Don't hesitate to ask for some help
from her doctor.

It takes a special sort of person to deal with this illness.   If you fear
your girlfriend will leave because of this, you may want to consider placing
your mom in an Assisted Living, or some other arrangement.