I think you might have to bite the bullet here and choose between them Personally, I am of the belief that in a longterm relationship that it be treated as the priority. If you are unwilling to do so then your choice is obvious.

I like that term 'enmeshed'. It seems to describe very well what is happening here.

Try to choose your times to communicate with your mom for when your girlfriend is present. I am serious in this. It will go very far to develop in her mind that you are not going behind her back to interact with your mother. So too, I think it would actually help to include her whenever you see your mother too. Also, if this is happening, cut off all dependence upon your mother. Try to avoid situations where she does things for you.

These things actually helped me a great deal. It both reinforced to me that I was the priority and reinforced to my now ex mother-in-law that I was part of the package.

It took some time but she eventually realized that I was part of her son's life - and hers too by default.

She never came around to getting excited to see me, but she and I did come to being on rather pleasant termss. It is ironic. Now that her son and I are no longer together we still will get along very well in those social situations we are both present.