Betrayal Trauma?

Dealing with the repressed feelings from betrayal in my relationship which is making it hard for me to see my partner involved with someone else due to past experiences… the other person is a wonderful person and not the issue… it’s my partners past mistakes that is bringing up insecurity within myself. Who has been through this and how can I get over it. We’ve been together 20 years and the betrayal is more in recent years. We both have partners we deal with.. I don’t want this to affect our relationships outside of each other.