You probably won’t like this but...
Your relationship with him seems very unstable if you are unable to acknowledge and celebrate his past as well as his present. The reasons for the breakdown of his marriage have no relevance to you (unless you were one of the reasons). He has children and she is their mother. They were married, they shared a house, they slept together, they shared the joy of the birth of children, they shared friends and jokes and meals. He remains, therefore, in a relationship of sorts with her, and you can’t simply obliterate her from his life. Be glad he chose you, be thankful to her for helping him to become the man you love, and ditch this teenage jealousy thing before it starts to poison the good relationship you have now.