Amor, you do seem to be beating yourself a lot for fucking up horribly. I'm still not totally clear on your situation here, but... can I check some details here?
You were married to your husband, right, when you developed a friendship with another man. Over a period of (months? years?) that friendship turned into love. Once you realized you loved this other man, you kept it a secret from your husband for awhile because you didn't know how to tell him and you didn't want to hurt him. How long did you keep it a secret? A few days? Months? This does sort of make a difference.
I think you didn't do anything physical with this man, and we're just talking about an emotional love?
It sounds like you eventually did tell your husband about your feelings--or he caught you and confronted you? Again, this makes a difference. And then you subsequently cut off all contact with this friend, right? Was that something your husband asked you to do, or did you do it to manage your own feelings, or both?
Pending more incriminating details, it looks to me like you did everything RIGHT, not wrong. Your husband's hurt because you love another man--and he should be hurt by that--but if my reading of your story is correct, you didn't "fuck up". You fell in love (it happens!) and then you mustered up the courage to tell your husband about it and then cut off contact with the object of your affections. What else were you supposed to do?
My point is that I think you should stop all this self-flagellation!
I guess you could say, "Well, I shouldn't have fallen in love," but you can't control your feelings like that. Nobody can. You could say, "I should have stopped seeing this guy sooner," but that is hard to judge, too.
Love can develop gradually, and sometimes it can be like cooking a frog: you start frogs off alive in lukewarm water, and gradually warm it. They don't jump out if you warm the water slowly enough, because they don't notice the exact point when it's become too hot for them to live. So they die in boiling water, never having jumped out. You died in boiling water, Amor. At some point, you were just friends with this guy, and then it warmed slowly and you couldn't tell you were boiling until it was too late. Right? And then--oh god--you were already boiling and you've already "fucked up" and you can't take it back, and how do you tell your husband that if that means you'll lose everything?
I've been there, Amor, I have. I mean, I actually did sleep with the guy, and my husband reacted differently than yours when I told him ("Oh, honey, you cheated? Thank goodness; I thought it was something serious!") but I understand the sick feeling in your gut that you have lived with for however long you lived with it.
So sure, beat yourself up if you want, but... also, take my sympathies, and my assurances that you're not the worst person in the world and that you've been hurt too.
That said, the question on the table is: can you see the guy again? You want to see the guy again, because you love him, and ripping an attachment apart like that hurts a lot, and gluing it back together, even imperfectly, can be comforting. But your husband doesn't want you to see the guy? Have you actually talked to your husband about that, or are you just assuming he wouldn't be cool with it?