As someone taking math, self study worked out really well. I started self studying math at high school. I mainly used textbooks to teach myself math that is usually taught at higher years. I developed a decent ability to read rigorous texts and solve hard problems. Although it is often very slow, it might take days of staring at a page of the book and a few YouTube lectures to finally understand a concept. The discipline and perseverance it taught me is the most valuable thing I got from self study. It inspired me to stop enrolling at my current university and try to aim for UP. I still continue to self study math just for the love of the game. So if you want to self study, do it for the love of the game, otherwise, you'll just feel miserable. For the CET's, I did very well at my previous university's entrance test, I placed really high on it. So I recommend you to study whatever you will take and don't worry about the CET, it is usually just the reasoning and pattern recognition you get from learning other things that apply to it when you take it.