When you are engaged to be married there are two types of doubting you may have regarding if you have found the right one. One is an alarm bell that should not be ignored! The other is a sign that you have made the right choice…
The great Kabbalist Rabbi Levi Yitzchok Schneersohn wrote a letter to his recently engaged son (printed in Likkutei Levi Yitzchok Igros Kodesh p202), explaining that engagement and doubt go hand in hand. The union of soulmates is such a lofty and super-rational event that the mind cannot possibly grasp it. Something is at work that is beyond our understanding and therefore there is always an element of doubt.
This doubt is not an indication that you have done something stupid. On the contrary, it means that you have touched a level that lies beyond the confines of the human mind. Finding your soulmate is such a miracle, our logic cannot process that it is really possible, that it is really true, that I have found the one for me. This wonder is a kind of positive uncertainty – Is this for real? I don’t believe this is happening to me!
So the fact that you feel unsure may be a good sign, indicating that you have been touched by the hand of G-d, and are left in wonder that it could really be so good.
Menachem Mendel Bluming, Rabbi Moss and Chabad.org