You suffered from a classic problem in Love; the
assumption and belief in the linear flow of Love as accordingly to
the 'standard progression', in which a relationship will graduate
into a marriage irrevocably.
Many couples talk about marriage - surely, when the relationship
seemed to have progress beyond a year or two, such conversations
are typical, especially when one has reached a 'marriageable age',
as well as having other aspects of their life pretty much in
control and stable (career, finance, etc).
However, these are but human speculation of what they think will
happen - or more accurately put across: what they want it to
happen.
Don't get me wrong; I am not suggesting that people shouldn't talk
about marriage, but rather, our perception has been tuned to such a
way that we are unable to accept any U-Turn policy that goes
against this linear flow of thoughts.
Someone having the same set of problem asked me the same question
before. I just told him this: 'Unlearn what you learn in
Love previously and relearn everything
again.'
He looked at me with this blank look, thinking that I was crazy or
something.
I will tell you the same thing; however, in addition, I will
explain why. 
***
Love is freewill; true love cannot be forced upon
by threat, violence, demand, anger, abuse, terror, fear, guilt or
habit. If it does, the relationship will no doubt be intense and
unfulfilling.
And you know what? This is but the very rudimentary understanding
in Love - that we all must see the logic, in which Love is a choice
and the choice made to stay in a relationship must be attributed to
the individual's freewill. If not, misery will follow suit
closely.
Therefore, what if your woman's choice is the desire to be 'out',
despite wanting to be 'in' previously?
Would you still see the same rationale?
Chances are, you can't really answer me - because there will be
fear in agreeing.
Love is never stagnant; like dough, it constantly remakes and
renews itself. You believed that a change in environment brought
about the change in her, thus the change in your
relationship.
But the truth is
that Love is an inside thing, not outside
job.
If an environment could so effortlessly affect the Love in a
person, it basically goes to suggest how enervated and languid this
Love he or she claims to have. Or there might be litany of unspoken
grievance or frustration felt within her, causing severe friction
as it ran through her emotions for many years, unattended and
uncommunicated.
Basically what I meant is that you know next to nothing about the
sudden drip in your relationship. All these years, you probably
assumed that you know what's going on, both inside and outside, and
that everything is marching towards to the direction you have point
out so prominently.
Your assumption in your relationship presumes that you are a loving
couple.
Your assumption in your relationship presumes that she will always
spend all her time on you.
Your assumption in your relationship presumes that she will always
want to marry you. Never mind the future - her words will guarantee
the future commitment as well.
Your assumption in your relationship presumes that you are the one
for her. No one else has the prerogative to that.
Ironically, if your assumptions are all right - you won't be
posting here.
That's probably the cusps of the problem; you think you knew your
relationship, but it appeared more like a stranger to you when it
blatantly placed right in front of your face and you suddenly
realized you didn't recognise the shape, which you have always
reckoned it to be your Love.
If you do not unblock this linear thought and relearn to love your
relationship, like a child, all these assumptions will continue to
widen, like the income disparity between the rich and poor in
Singapore.
You relearn by:
I) Eliminate your linear mindset - Quit pressuring her with this
marriage thing. Quit thinking that this relationship is already
years in making. Quit thinking that your have invested far too
much. Quit thinking that you are unable to live without her. Quit
thinking that Love is irrevocable.
II) Communicate with her, as if you have never spoken before. She
must be comfortable to speak to you without fear and you have to
find some means for her to be able to exude that level of
frankness. Basically have a heart-to-heart talk. Minus
guilt-pressuring. minus overly emotional speech, minus finger
pointing.
III) Understand your relationship from her perception. Enquire from
different angles and seek to understand the struggles, which are
not transparent in your vision, which might have taken place
beneath your nose. Learn to learn your relationship from an
alternative perspective and not view it like the way you always
do.
You can only move forward, if you have establish the fact that her
decision is still a 'yes' to this relationship - it must not be a
diplomatic yes, but straight from the bottom of her heart. Then you
can seek to address whatever problem that is degenerating this
relationship and remove the source.
But if it isn't the case, then you might want to ascertain how much
damage has been done and if there's a possibility that you can
rectify the problem and overcome it. 
Cheers