This really doesn't have anything to do with him or his drug use or what he did while under the influence.
It has everything to do with YOU and YOUR self esteem. Are you the kind of woman who doesn't mind being cheated on? Lied to? Will accept excuses(drug use) to write it all off?
Addiction is a life long battle, rehab is only the beginning - a chance for him to walk through a door to live a life clean and sober with honesty. His history has already told you he can't maintain that life style - the drugs, the woman will always be calling his name and he will always be one bad decision away from picking up again.
When we are in pain when a relationship has ended and we can only see and feel the love from the beginning of that relationship, having them come back to us promising us there love and devotion - we jump and accept the unacceptable because we want OUR pain to stop, we want security for our future.
The fact you are stepping on egg shells around his sobriety, not wanting to rock HIS boat, means you have already placed yourself in the back seat while he drives this bus.