i have to tell them its one of their friends

Oh, well that changes things then. If you're having sexual thoughts about one of your partner's friends, maybe you're in the wrong relationship. All in all, you can't really help you're attracted to and someone's going to get hurt eventually, whether you tell him, it becomes blatantly obvious, or you continue to bottle it up and it puts a mental strain on you.

If it's just a passing lust, then it might go away. But after looking at your post history, you posted about the same thing 4 months ago and your desires haven't changed. At this point you've been emotionally cheating on him for a few months, no wonder you feel guilty about it.

If you really love your partner, you should probably tell him the truth about your feelings for his friend. If after 4 months your feelings for his friend haven't changed, and you're feeling guilty about it, it's unlikely it's going to change. Staying with your partner while actively maintaining lustful fantasies for his friend isn't fair to him.