First, Well Done for not drinking.
I have spent time with people with whom I used to drink. Not saying that's right for you, but I found that when I honestly stick to my guns about not drinking, it gives me a sense of accomplishment.
I've just said, "I quit drinking." When they question me I say, "It doesn't agree with me."
I've left situations where my friend has become intoxicated. That's no fun having to deal with a drunk person, so I excuse myself. As long as I'm honest, I have no ill regrets.