NOTE: My answer is not specific to snooker, but attempts to provide insight toward what, I believe, you are experiencing.
Consistency comes from practice, practice, practice.
As a golfer, I have experienced performances you have had in snooker. Some days, I'm playing well. Other days, not so much. You have been playing snooker for 4 months. I have been playing golf for 15+ years.
We're sitting here talking about practice, not a game. During practice, you get an idea of what you have to work with a given performance (akin to "one day I can see clearly the point I have to aim at ... the following day I can't visualize the shots") and an opportunity to further understand how you perform.
If you are in a less-than-favorable situation (in your case, can't visualize the shot), practice will help you mitigate it. Mind you, mitigation doesn't necessarily mean "strive for peak performance," but it will help turn an unfavorable situation into something manageable and turn a potentially bad performance into something better.
To be more consistent overall, practice will give you the opportunity to continue to do what you are doing well, and improve on what you are not doing so well. You will never arrive, because what you are doing well now and what you need to improve on will change and swap over time.
- Understand your style of play and embrace it
- Learn how to mitigate less-than-favorable situations
- Continue to do what you are doing well and improve on what you are not doing so well
Hopefully, with these points, you can be more consistent in snooker (or any sport you're playing in).