He was much less concerned with getting good grades in his classes than he was with simply getting passing grades. To him getting a D in a class meant that he was finished with it and he could move on to the next class.
Now, don't get me wrong, Greg was a smart guy. But he had different priorities. He was there to learn what he needed to learn in order to go out and get a job. It was a means to an end. He was not learning for the sake of learning.
Look at your marketing efforts in the same way. You are not doing marketing for the sake of doing marketing. You are marketing so that you can further your art career.
So, rather than worrying about creating the perfect newsletter, just create one and send it out. You can always improve on the next one. Or, rather than stressing that you don't have a mailing list, start collecting names at your next show.
Marketing is a long term process. Sending out one perfect newsletter and no more isn't going to get you anything. So, even if it's not perfect, just get it out there. Your customers need to know you exist,
As far as marketing goes, you don't need to do it perfectly, but you do need to do it enough.