After thousands of hours on the lesson tee, this is probably the most common "Self Diagnosis" that I hear from new students. I completely understand that you are contacting the ball either thin or heavy, The fault is NOT a premature lifting of your head or the inability to "Keep your eye on the ball".
Your friend's good intentioned fix - "Keep your head down!", is one of the most detrimental to solid ball striking.
Pictured are two professional golfers who have posted tournament rounds in the 50's. Observe that neither are keeping their head's down or looking at the ball, pre-impact. I could add many other examples as well.
The problem is your swing radius is inconsistent. There are a myriad of reasons why this is occurring. Like finger prints, we all have our individual tendencies and patterns. End the guessing and frustration. Get on a Plan to Better Golf! We will assess your swing and assign drills for your improvement.