I am married with three kids and my husband has a very close friend whom I have been admiring for the last seven yrs. But nowadays I have a feeling that I am going crazy I can't even eat neither sleep just thinking of him every moment. Please hejp me. I seriously need him next to me.

You want him, which is very different from needing him. Are you prepared to face the consequences of a life with possibly neither man and no children? Does your husband's friend want you? What would he do to provide, or do you have a good job? Will your husband sue for custody of the children? Will he take you back if it fails after the first romance period is over? Have you actually given this any thought?
Go see a therapist about what is behind this. Yourself indulgence could be the ruin of several people.