Your shooting routine is a habit that you perform at the table, and you can improve your consistency with feedback and structured training. Feedback tells you what to improve and training is how you do it.
Feedback is crucial for pointing out your inconsistencies. When you know where you need to improve, then you can tailor your training towards this.
The easiest method for feedback is to shoot a striped ball straight up and down the table. Any sidespin will be visible as a wobble on the ball and also change the ball's direction off the rail.
Another great way for feedback is to record yourself shooting. You can then watch the video to spot inconsistencies in your routine. Keep all video recordings of yourself so you can see how you progress over time
The best feedback mechanism is to find an instructor. A good instructor will be able to immediately point out elements of your routine that are inconsistent and movements that could hinder your playing abilities. A great instructor will also use video feedback to show you.
To make a change in your routine, you must first consciously and deliberately be able to perform the change. Sometimes this means moving your body very slowly to ensure the old habit is overridden by the new one.
Start with rolling a ball straight up and down the table as described in the feedback section above. This will let you get a feeling for hitting a ball, but without the strain of trying to pocket anything.
After about 15 minutes of this, progress to a short straight-in follow shot, then a long follow shot, a short stun shot, and finally a long stun shot. If you find yourself performing the old habit, then slow down!
Train like this a few times a week and after about 3 weeks the change will become more automatic. Your playing abilities will temporarily be hampered during this transition, so be cautious of playing in competition.
Keep in mind the long-term goal of increased consistency leading to fewer missed shots, and training can become fun!