Hi Donna and Welcome.
18 years is a long time. I have nothing really to add to the wise words already posted. I am agreeing that you have not failed the marriage, he has by choosing to relapse.
You are the one who may be faced with making very hard choices in the future. I am sorry for your having to suffer that.
Often the spouses and significant others of addicts are told they are crazy and lying by the addicts and the addict's family.
It is helpful to start your own program and your own recovery here and at AlAnon and/or NarAnon meetings so you can have a place to go where everyone knows you aren't lying and are not crazy and for you to know you are not alone.