do you have toe clips, or straps or clip ins? if your just putting your feet on the pedal unattached your only using half of your strength, plus your foot might just slip off. Use straps or clips, when you lock one foot, use your opposite foot and pull up. this will help to stop the back wheel. Also shift your weight forward and get out of your saddle. if your sitting down, all of your weight is on the back wheel making it harder to stop. Don't bike in a grass field, ride on cement or pavement.