A few thoughts for you mm...1977:
A year ago when I first came to this forum my outrage at your story would have made it difficult for me to write anything calmer or more reasoned than Raze"s "Sl*t" post. But I've learned a lot about the realities of Modern Love since then. Now, "How typical" pretty well covers my reaction.
These days when I hear some poor guy screaming "She cheated on me" all I can think is "Of course she did, Fool. Did you really expect anything else?" The majority of American women are on your side,mm, or on their way to join your cause: around 50% of women in this country cheat on their "love"-partners, and another 25% say they would cheat if they were sure they would not get caught. You have plenty of company out there in Adultressville, though not very distinguished company. And I no longer give a damn that that's the case.
So I'm not being "judgmental" here, only pointing out what I see as the realities of your situation.
1. You say that you "have only been intimate once" with your husband's "friend." Too bad. If you'd had a hundred "intimate interludes" it wouldn't make your husband feel much worse than your one-night stand is going to, and you would have got a much better price for your marriage.
2. You say that you're your husband's "best friend." No, you're not. Perhaps you once were, but you are no longer, nor will you ever be again. Don't make decisions based on your supposed friendship with a man you castrated.
3. You say that you're "afraid of hurting my husband," as though the hurting is a future possibility. I'm sorry, but you're not thinking when you say this. You've already hurt your husband. The pain may not have reached his brain yet, but it will the second you confirm his suspicions (yes, he probably does suspect), or he has his suspicions validated by any of the many, many people who know of or will learn of your little affair. Count on it, because you'd be a fool not to.
4. "We recently ended the affair"... No you didn't. You've got your "friend" on standby (as he has you), and it's only a matter of time before you and he (or you and somebody) are off in some hourly-rate motel, harassing the bedbugs. I'm sure that if you look deeply into yourself, you'll see that I'm correct. Admit this to yourself, and you'll start thinking a little more clearly about your situation, and about whether you want to keep your pitiful husband around to endure yet another round of humiliation.
5. When you say you need "a way out of this mess" I assume you mean that you want to save your marriage. But...you no longer have a marriage to save. You've destroyed every root, branch, and leaf of any "holy matrimony" you may have entered into, and you've broken the legal union/civil contract marriage as well, as any divorce-court judge will tell you. Nor do you any longer have a "relationship," except as cheater to cheated. Your husband may still think there's a love relationship, but that belief is not likely to last past revelations about you and his "friend." Perhaps you and he can develop a new relationship in the future, but the cards are against it. Fewer than a third of cheater-broken marriages survive more than months after the cheat is revealed.