Hey, in advance, I don't have adhd but my significant other and a very close friend have, so I just want to give my 2 cents from the other side.
Don't feel embarrassed or guilty if you didn't message for some period, it's not like you're doing it on purpose. Don't hesitate to speak out this topic and try to explain that's nothing personal, some might get it some not but it makes contacting them a lot easier.
Some personal advice: I know it's hard to track but try to contact them when you have different moods. Its hard to be polite, understanding and not annoyed if your friend is only contacting you when he/she feels miserable.