How do you not have your confidence shaken up when in a relationship with a partner who was truly in love with their now ex and only broke up because they couldn't have that ex?

Being in a serious relationship with someone who dislikes their ex seems so much easier and non threatening but maybe that’s because I’ve never done it before.

For people who’ve gone on the 2nd, 3rd, etc. marriages/serious SO’s, how did you get past the feeling of not being the number one choice of heart? Did you look at your partner one day and just know all of sudden everything was okay and that they really loved you?