It is a load of cr-p. He is not sober. Even if there is nothing romantic / sexual going on with the OW (I'm not saying one way or the other), him going out and drinking with her when he knows it leaves you feeling bad is just sh-tty. That's not the action of some one who wants to or knows how to be in a committed, loving relationship. You deserve to be treated better than that.
It helped me to not look at it as forever. I just had to not see him, call him or take his calls today, the next hour, the next few minutes. The more time I've been away from him, the clearer I've been able to see our relationship and realize that it was not what I wanted or needed; who I was trying to hang on to was not who he actually was, but who I wanted him to be.
Sending you hugs, JW123.