I think a good cover and title are important to draw attention to your book among the 100's/1000's out there, but there are so many good sites and artists out there who make beautiful covers.
Don't try to perfect your first draft, just get your ideas down and worry about editing later. That's what the editing stages are for.
On those off days, don't beat yourself up about not being able to write, or if things aren't piecing together. Allow yourself to take a break. Work on a different project, or have a few days/week off writing and come back to it.
Sometimes a quick break, or a different focus can help you get past those hurdles.
And;
Don't give up! Keep at it. Even a few words a day can help you improve and get you into the habit of writing daily.