Well, even if the one you love is falling/has fallen in love w/ another girl, I won't think it matters at all, because love knows of no boundary. So goes the old saying: "You may not love me, but you cannot stop me from loving you." Remember, as long as you respect his choice, you can be loyal to yourself at the same time.

Thomas

(PS If you, unfortunately, really encounter something like this in future, pls NEVER ever blame the other girl, for she hasn't done anything wrong. Also, you may try to de-emphasise the relationship by, e.g., treasuring the friendships you already possess.)