"もし同時に二人の人を愛しているなら、二番目の人を選んでください。本当に一番目の人を愛していたら、二番目に惚れることはなかったでしょう。"

Simplified Meaning:

If you find yourself having feelings for two people, think about why you now have these feelings for the second person. If your love for the first person was truly strong, you wouldn’t be attracted to someone else. This suggests that your feelings for the first person may not be as deep or genuine as you thought. Imagine you have a favorite book that you read over and over because you love it so much. One day, you start reading a new book and find it even more exciting and captivating than the first. This new interest shows that although you liked the first book, it didn’t completely satisfy you. In a similar way, when it comes to relationships, if you're really committed to someone, there won't be room for another person in your heart. So, if you find yourself drawn to a second person, it might mean that your connection with the first isn’t as strong as you believed. It's better to be with someone who completely engrosses your heart and doesn’t make you question your feelings for them. This advice helps you find a relationship that's more fulfilling and real.