I have the type of ADD that makes me shut down like that. And I do understand how your friend may be feeling. That totally does not make it okay. If I ignore and hurt my friends because I can't handle everything going on in my life then I do not deserve to have friends. That simple. I don't want to be like that. That's part of why I'm in therapy. It's a very simple thing to send a text letting the other person know that you haven't forgotten them, you're just stressed. You are working hard on your issues. You deserve better than to surround yourself with people who are not willing to work on theirs.