I have always taught my golfers that every putt is make able if the speed is correct, every putt is a straight putt and the first 3 feet of every putt is all a golfer can control.
A lot of golfers either push or pull the putt off line because of either an in-to-out or out-to-in path direction. A great drill to help develop the proper path direction is to find a hole on a flat area of the green. Place either 2 clubs or 2 alignments sticks 3 feet in front of the hole creating a pathway to the hole about the width of the putter head. Make a putt so that the putter head stays between the 2 clubs or alignment sticks and roll the ball toward the hole. If the putter hits either of the clubs or alignment sticks your path direction is not taking the putter shaft down the target line. If the ball hits either of the clubs or alignment sticks then you are not impacting the ball with a square putter face. If the ball comes up short or goes past the hole, you are typically hitting the ball instead of rolling the ball. Make sure the putter head stays low to the ground and follow through with a high finish to create true roll.
Once you become proficient from 3 feet, back up the 2 clubs or alignment sticks in 1 foot increments and putt again. The goal is to develop a putt stroke that starts on the proper path direction toward the target while developing speed control that rolls the ball into the hole.