I've been with my partner for 17 years and it has never been the all-consuming, butterflies, weak-knees, finger-tingling relationship like the one prior to this....
That particular relationship (3 years) was passionate in all respects......but I was as unhappy as I have ever been whilst in it.
The current Mrs. JtH is my life-partner. I adore her and will grow old with her. My heart swells at the thought of her; the best sound of the day is that of her key in the door after work. But no, I don't get the symptoms of 'passionate' love and I'm grateful for that.............it was exhausting and I'm no longer prepared to trade-off the lows for the highs.
I'm not sure that I have explained it terribly well but I think you'll be surprised at the depth of feeling you actually hold for your new girl; it just doesn't feel as 'strong' when compared to your past love...........
Stick with it but if the doubts continue to grow, then perhaps she isn't right for you and only you will truly know that and when.