Pre-sanctified Liturgies occur only during Great Lent so one, if he is following the rules of abstinence, should abstain from meats and dairies. And your priest is right, fasting was not instituted to really prove ourselves to God by possibly causing ourselves serious bodily harm. Like any other discipline, it must be eased into. And, for many, it is not easy. I would heed your priest's advice and eat a light breakfast and a light lunch, making sure you get what you need and then fast until the Liturgy that night. But, remember, if your fasting is unaccompanied by any prayer during the rest of the week, your fasting will avail you nothing.