On the day my husband marries his new wife, how do I manage my feelings?

Plan that day in advance so you do not face it with an open, unprotected heart. Decide early how you want to get through it: with supportive family, filling the time with calming activities, or keeping away from things that inflame the wound. You are not obliged to attend or celebrate it if that is too heavy; protect your heart first. Avoid following news or social media about the ceremony throughout the day. Do plenty of dhikr and du'a to calm your heart, and allow yourself to rest. If tears come, let them flow; this day will pass. The next day, take one more small step forward. See mental health.