What does it mean to stop looking over your shoulder? Well, if you're in a relationship, it points to mentally comparing the current relationship to past ones, whether they were lousy or wonderful. Either way, it prevents you from being objective about the current situation.
Within a relationship, looking over your shoulder refers to times when you're in a conflict with your significant other, and instead of dealing with what your feelings are at the moment, you spend all your time pointing out situations in the past to prove your point, a point that may cost you dearly in the relationship. Few relationships survive score-keeping.
Within your own psychology, looking over your shoulder refers to an emotional attachment to your past life experiences. If you are afraid to move forward, whether it's personally or career-wise, because your past experiences have never worked out well, then you are short-changing yourself. You should, of course, intellectually assess what you may have overlooked in the past or what attitudes may have stood in your way. However, this should be done with the intent of moving forward, not wallowing in the past. This means that you leave the emotions attached to the past in the past.
So what are two important things to remember about this? 1) Make sure that all your thoughts regarding past experiences are viewed in terms of how they can best serve your current desire; 2) Stay clear of the negative emotions (anger, poor-me's, etc.) that are attached to your past experiences. This will not serve you in the present.