I think "dont go to bed angry" doesnt mean finish the argument and be happy as much as "dont go to bed while youre having the argument" i think by "dont go to bed angry" its more that you should stop talking for the time being until you are thinking about something unrelated(technically not angry anymore) then go to bed and rehatch the issue at a later date. If youre arguing and go "fuck this im done talking. Im going to sleep" and going straight to bed. If you do the former then youre still laying in bed poutting to yourself about the argument. Its vest to say "im done talking right now" go distract yourself and go to bed so you do fall asleep dwelling on the argument and being angry.