Have you considered that maybe your girlfriend could be suffering from mild depression? A lot of the things you've mentioned (weight gain, lack of interest appearance, not enjoying things the way she used to) are symptoms of depression, and it could be that your girlfriend needs a bit of help and support rather than criticism.
However, it could be that your relationship is not what it once was and this could be a sign that you both need to sit down and talk about it. It's very frustrating when one partner feels that they're putting more effort into the relationship than the other, but often the other partner doesn't see it that way and so you get a conflict. Better to sit down and talk things through rather than simmering with silent resentment and getting to the point where you dislike each other.