Hello paddywoo. You have had lots of good answers, particularly from Miss Zippy. When you know you just have to say those three words because nothing else you have said describes how you feel, then say it.
2 weeks after I met my (now) husband, we were saying goodnight (at this point, we had had lots of 'kisses and cuddles' but nothing more). He said to me 'I wish you were my wife' and I said 'I wish I was too'. We were both in our late 20's and had had a couple of long relationships before, but this felt different for both of us.
We went on holiday to a beautiful Greek island when we had been together for 11 weeks and got to know each other VERY well... On our second day, he proposed, I accepted (thinking about time!) we married 14 months after we met. We have now been married for over 11 years, and have 2 children aged 1 & 4.
All I can say is, go with your heart, but try and read your girlfriend well too.
The very best of luck to you both.
P.S. Sorry to have waffled and sounded a bit Mills and Boonish, but it still gives me goose-pimples thinking about how we met!