How will you deal with your girlfriend turning 40?
Change in relationships are pretty much the only constant in relationships*.
The best way to deal with changes in your partner over which you have no control is:
- Introspection – figuring out why the change upsets you
- Communication – using Nonviolent Communication techniques to explain your feelings to your partner
- Negotiation – finding a good "middle road" that allows you both your personal freedom to grow as individuals, while walking the same path together
- Resignation – letting go of the need to "win" the negotiation phase, and to not only tolerate, but move on from whatever outcome you can both agree on.
If you can do those four things effectively, you should be well on your way to resolving just about any issue you'll face. Just how to establish and conduct these four steps is the subject of a much longer post, though there is literally a few metric tons of information available in the printed literature: http://amzn.to/g3iD71
*Also, be open to exploring the other constant of relationships – the ending. If you can see how you'd like it to look in order to be considered a "success", and see how different/how far off that looks, you should have a much easier time weathering whatever immediate storms stand in your path.