I’m not a big painter so after reading up on it a bit, I took the following advice:
“Paint into all the corners with a 2-inch or 3-inch paintbrush. Use the same brush to outline where the ceiling meets the wall (and vice versa), around doors and windows, above the baseboard and around any other trim or detailing – and wherever a paint roller won’t fit.”
I actually put 2 coats of primer on. But now I can see a 3″ outline through out the entire room where I used the brush at the corners. to me, it looks like 2 different shades of primer.
Did I mess up? Will it be covered up when I actually paint?
No you did not "mess up". Yes, it will be covered with the finish.
A couple of pointers Karl has covered in his articles. Try to maintain a "wet edge" as you paint. In other words, if painting alone, do not "cut-in" ("Painting into all corners…) all the walls and then come back with the roller. Cut-in one wall and roll that wall before you move to the next. For us, this is not terribly difficult as our cut-in time is not long. For you, you may want to enlist some help in order to lessen the possibility of the "outline" (what we call "hatbanding.")
Always remember, primers are not finishes and will not look as good as a finish. That is not their job. You are on the right track.