You are on the right track.
Sorry that your husband is in denial and not trying to help you. It is like hearing the fire alarm and trying to wake him up, but he is convinced it is fals alarm and wants to keep sleeping.

At least you are awake.

Although I agree, that it is much easier when you have no contact with this man, this seems to bee difficult to achieve at this point in time.
There are things you can do though. You do not have to hang out with him. If your husband invites him, go to your sport studio and work out. That way you will not only look fantastic, but you will not be seeing him. (or go visit a girlfriend.)

I would also reccomend, that you tell the girlfriend that he has beene-mailing/texting you and that it is beginning to affect you in ways you don't like. You can show her the texts, so that she knows, and you can tell her, that you have decided you want to have no contact with him anymore, and consequently won't be spending time with her anymore. (if you didn't tell her this in any no-contact letter you wrote.)

Try to spend as much enjoyable time with your husband as you can. You know, it takes at least 6-8 weeks trying as hard as you can to reconnect. It may take a bit longer in your case, because of not-ideal contacts of your husband with OM.

Just tell your husband, he is right, and he is a better man for you than OM could ever be. You know deep in your heart, that it is true, and that you deserve better, than a man who is sexting another man's wife between his girlfriends and 'best' friends back.
Tell him also, that you want to get over your childish crush on the OM, and that you will not be parttaking in joint outings anymore and insist that he does not speak about him anymore. He wants you to be happy, right? Then he should at least keep his mouth shot.

Of course it is tricky if the other man is only an arm's length away, but you must try for the time being, and maybe work out another idea in the long run, when you and your husband become closer.

1
You should change phone- and e-mail numbers, or maybe have a joint e-mail-account.

2
Try to treat this to yourself as an addiction. Sadly, alcoholics don't get to live in a alcohol-free world, but many make it notwithstanding.

3
Can you expose yourself to some friends or familymembers of you, so they can give you back-up when you are tempted?

Women generally help their husband when he is feeling low. The sad thing is, not many men are emotionally equipped to deal with a woman who is feeling low. And research even suggest that they become LESS emotionally available and less pationt when their wives are struggling. (Sorry to all the men, I do not want to generalize, I know you her on MB are very consious of your wives feelings.) He probably never learned how to react properly and it totally in the dark.

You are absolutely not a lost cause. You will have to work harder for the time being. And if you have a setback, don't give up. Just stand up if you fall and keep going. You seem to be a strong intelligent woman, even if you don't feel like it.

Exposing was not a bad thing, but the girlfriend needs to know, to what extent her boyfriend whas pursueing you.

Keep going and start making your marriage better.
Good luck and God bless,

Happyheart.