You stop feeding that crush and tell him to leave you alone. If you had nipped this in the bud, it wouldn't have gotten to this stage, but it's not to late. Nip it now. Spend more time with your husband, you must have married him for a reason.
And then take this as a lesson not to allow this to happen again. Think about it, if you'd stopped him right away, would you be feeling this way now?
If you want out of your marriage, then get out, but don't cheat and lie. You can't have your cake and eat it too, not without hurting a lot of people, and your husband is already on to you. This is your wake-up call to save your marriage, I would urge you to take it. Also keep in mind that this other guy has no problems pursuing a married woman and cheating on his own wife, is this really the kind of guy you want? He sounds like a sleaze bag to me.