Wolfgaze, that is very insightful and a great way to explain it! It actually even helped me to understand some things better, so thank you.
I think another key factor here is to work on your self-esteem and feeling more confident that your partner loves and wants YOU. These women are triggering you because they may be prettier and party like he does, making you feel "less than" them.
The thing is, you are not less than. He is with you for a reason and says he doesn't need or want a party girl. Trust what he tells you and trust that he likes you for YOU and all that you bring to the relationship.
There will always be someone more beautiful in the world, but beauty is not just skin deep. Beauty goes so much further, which relationships require in order to be sustained. Have faith in yourself that you are beautiful to him, and a wonderful partner just as you are.
Because others cheated in the past does not mean he will also cheat. He is an entirely different person. The only reason to worry about that is if he is showing signs that he may stray, and it doesn't seem like that is this case.
((((Hugs))))