Hi Pegasus. Welcome to this site. I have been clean from crack for about 3 years now. Crack is a nasty drug. What your husband is doing isn't considered relapsing. It's called active use. In order to relapse, you have to actually quit and stay quit for a significant amount of time.
Crack addicts are very resourceful. They will continue to smoke crack until they feel consequences so bad that they are forced to stop. They will abandon their children. They will lie. They will do anything for more drugs if they are on a run. It's disgusting. Crack leads to jails institutions and death. Thats it. There is nothing else. Tough tough consequences are the only thing that will stop them from using. And even then it isn't always enough.
Your husband will be fine and he will continue to do whatever he wants to do - be it recover or continue using and "relapsing". You are left to be responsible and make all the tough choices and take care of the kids. Its tough. But it is what it is. Please put your children before your husband and his addiction. You are the only sane parent they have right now.
NOW is the time to set serious boundaries for yourself if he uses again. I suggest this one:
I will not allow crack in my life, in the life of my children or in my house. Therefore if anyone comes around me that I suspect is using crack, I will use whatever force necessary to make them leave my house, including calling the police immediately.
PS. You say you can't forget the past. Well I'm here to encourage you that you shouldn't. You should learn from it. He's been "relapsing" for years. He's been through treatment before. You've always been there to support him. He's never gotten better. We have this saying in the program... if nothing changes nothing changes. I don't mean to be a negative Nelly, but only time will tell if anything will be different this time. But hopefully if he relapses again, your boundaries will be firm and you won't allow yourself to be a part of his disease any longer.