I think it is a mistake for one person to live another's life were it possible.
To put it another way desirably you each have lives as individuals and some life you may choose to share.
To say the cruising life is superior to all others is a bit over the top. Some of its attraction lies in contrast - if you are snow bound images
of tropical islands may be beguiling. Those on tropical islands have their fantasies too.
The reality is that few manage to cruise
on a long term basis for many reasons including financial, as at times other aspects of life intervene; education, career, children
, aging parents etc.
On a practical basis it is difficult to have a way of life as a single
goal that one's partner does not share. How invasive cruising has to be in a couple's overall lives is negotiable however. That is neither has to be the priority in an either or choice.
If no common ground can be found then one's views on the primacy of relationships come into play.
Most would probably accept that a good relationship adds an enormous amount to life in a cruising or any situation. Well desirably anyway - I am not sure how many put in the efforts to maintain them.
Whether a particular person or relationship is the one and only is debatable. Some form successive relationships with ease, others sparingly. One partner may seem indispensable but in hindsight a disaster or vv, and a new partner may seem superior at first but in reality they are just different in some ways.
I suspect that success lies in some basic compatibility and in an ability for each to foster the other while also respecting themselves and their needs.
Some working together is called for. The original poster says his girlfriend cannot see herself living in a small space and for more than a week at a time.
It is not clear just what steps he has taken to achieve his dream and in what form it might be realised.
However a start would be listening to the girlfriend and addressing her concerns as being where she is at this point in time.
Both her points have some validity. Even a 40' boat is a confined space for two people. Secondly after a week or so confined with another person even without fatigue and sea sickness
the desire for other company and conversation would for many people especially perhaps women be strong.
Fortunately this applies also to others so cruisers tend to be sociable.
I also think there can be a difference in the content of how people think about cruising. One may focus on dangerous cramped and boring while the other sees adventure and freedom. Perhaps your girlfrind may be more attracted by the travel and independence than the sailing. That might include different food
, cultures, art, people whatever. There may well be particular places she would love to go to.
Finally leaping from a daysail to a life aboard may need some time, just as a first date wanting to pick the names for your five children
may be pushing it a little.
You have room to negotiate just get a pre-nuptial charter
in the the agreement sense.