Re: the meaninglessness of life: our lives have the meaning we give it. Knowing there is no 'after' just gives those lives more meaning now.
Re: Guilt/doing wrong: human societies have been developing moral codes for as long as there have been human societies. Being able to figure out what works and what doesn't is part of our nature. Human history bears this out, with moral codes developing and changing through the centuries (something that would be unnecessary if we'd gotten a 'correct' code right from the start).
In any event, I didn't choose atheism just as I did not choose to believe when I was a JW. Being wrong is one thing. Deliberately lying to myself would not work.