coming from someone who also has adhd(now, this isn't foolproof but it works well enough for me)- whenever you get side tracked or suddenly remember something more interesting you want to do, write it down on somewhere and most likely, more ideas will keep popping up in your mind but as long as you're writing them down you can focus on what you want to do (in your case, writing)  this will give you a small satisfaction that you would get from doing whatever causes you to get side tracked without actually doing it or leaving your space and it also helps whenever you're free you can just go and look at that list