Bad bad bad idea to marry your first love. They rarely make it. There are exceptions. You don't know who to marry unless you've dated at least 10 people. That's my rule. Date at least 10 people before you get married, more is better. This helps you find out who is a better match for you. Besides, you are curious about other people, you need to satisfy that curiousity.
DO NOT GET MARRIED. If you have doubts, there's a reason. If he really loves you as a perfect match, he will take you back after you see other people.
I have a long time interest in psychology, specializing in relationship dynamics for 30 years.
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