Try to explain to her that by going to the doctor, they can look at either changing her heart medication, or add an extra one to help her memory (if deemed appropriate).
The go definitely wont think she is stupid, and the earlier these things are caught the better in terms of slowing any deterioration.
Before they even start diagnosing or thinking about dementia they do a blood test and ecg to rule out any other cause of memory problems - this in itself might reassure your mum that the go will look at every angle.
If it is indeed dementia, then she needs to know what she (and you) are dealing with - you will want to aim for more good says than bad.
My experience is both professional and personal; my grandmother is staunchly of the mindset that nothing is every wrong with her, grandad is far worse than she is, what's everyone else's problem. Nevertheless, we caught her on a good day, and I telephoned the dr to arrange an appointment to sit and kick start the dementia diagnosis. I knew it was due to my job, but we still went through the whole process etc.
As much as she kicked off about it all once there...she forgot about it. And she has some days where she is very self aware and knows something is wrong, but jot what it is.
Sorry, rambled a bit there.
Suggest that you are concerned about her, you want to make sure there isn't any other cause for her being a bit forgetful, and offer to be with her through it all. Offer to make the appointment - this may ease enough of her decision to agree to it