The best thing you can do for your painting is to step back. This isn't just art-school advice, I tell ya, it's true in every one of your life experiences and it's true for your art work!
You must step back over and over again with your work held completely straight to your view. You must see it from a distant perspective, make better decisions about your shapes, values, story. Every mark you make shapes the power of your piece. Every opinion you consider is valuable information to shape your destiny.