I have been meeting with a young lady for several months. Our relationship is very strong and it looks like we will decide to marry. Sometime I feel a need to tell her that I love her. Is this allowed?
The Halachic authorities specify several subjects, which "the souls of men covet." On the issue of taking interest on loans (Yoreh De'ah ch. 160) it is said that we must distance ourselves from interest "very very much." On the subject of bribery (Hoshen Mishpat ch. 9) it is said that a judge must be "very very" scrupulous not to take bribes. And so with regard to men and women keeping a proper distance from each other, it is stated (Even HaEzer ch. 21) that a man must distance himself "very very much" from women.
Our underlying principle is that the whole world is full of positive strengths. But when these are utilized in the wrong place or at the wrong time, then they become negative. For example, the love of money is the cause for taking bribes and interest. Our sages say, "Tzaddikim, their money is prized by them" (Sotah, 12). Therefore, the very characteristic of placing importance in money is positive, and it is considered to be a man's enhancement. However, using this power in order to exploit others by means of interest or bribery is "very very" negative.
The same goes for relations between men and women. G-d did us a kindness in instilling in us the power to love, especially the attraction between the different sexes. Without this attraction, we wouldn't take the trouble to establish a home, to marry and to bring children to this world. Our sages tell that in the days of Ezra the Scribe, at the beginning of the Second Temple period, the sages received control over the yetzer hara (evil inclination) for sexual relations, and they wanted to nullify it. But the yetzer hara itself warned them that the world will become extinct without it. Indeed, several days later, even cocks weren't attracted to hens. Therefore, the sages only weakened the yetzer hara, but didn't nullify it entirely.
And from here on to your question. The Torah of Israel, and in its footsteps the sages of Israel, knew how to direct human strengths. They determined that at a young age, it is not at all proper to give expression to the attraction between boys and girls. Afterwards, when a young man and a young woman come of age, it is a Mitzvah to make a Shidduch (introduction) for them, so that they can prepare to establish a home together. However, all the restrictions continue, absolutely and without exceptions, right up to the moment of the marriage ceremony. But from then onwards, the laws change beyond recognition. Deeds that were forbidden us by a severe prohibition, in one moment not only become permitted, but turn into a Mitzvah. However, all of this happens after the act of acquisition of "You are Mekudeshet (betrothed) to me with this ring..." completed with the Chuppah (marriage canopy), which is a supplement to the act of the Kiddushin (betrothal).
To sum up, before the wedding, up to the act of Kiddushin, intimacy with your fiancée is forbidden just as it was the first day you met. You cannot act with lightheaded casualness with her, and you have to distance yourself "very very much."
Immediately after the Chuppah (in privacy, not in front of all your guests!), it will be a great Mitzvah for you to say to your new wife that you love her.
A Suggestion for the Meantime: You can say to her that if it were permitted, you would tell her that you love her. But since it isn't, you can't say it...