I do not know whether affirmations and visualizations work. I do know that there is research showing that people who understand that mastery comes from effort over time have an advantage over people who think that talent is something you're born with.

This is true whether you think you have a talent or not. Obviously, if you think, " I'm not good at math" and just give up, you won't give yourself much chance to improve. If you always thought you had a talent for it, then had trouble, you might decide that you don't have any talent after all, and give up.

On the other hand, if you understand that you get good at something by working hard for a long time, then when you have trouble, you know that you need to work harder and find better strategies in order to succeed. So a good message to yourself would be, "If I keep working hard, I will get better and better at math."

Courage tends to come from doing things that you are scared of. Telling yourself that you are brave is probably not as effective as taking small steps to do things that are important but scary. When people say mean or unfair things about someone else, stand up for them. If you are nervous about speaking in front of a group, take a drama class. Then when the next scary thing comes along, you will remember that you have already faced fears and can do it again.

Having a good attitude and wanting to improve are good first steps.

Sometimes 'well known facts' are wrong, especially with research about people and our behavior, which is very complex.So I wouldn't take it seriously, that one needs to day, "I am excellent at math." I've heard the affirmation, "I am getting better and better, every day, and in every way." That seems too extreme to me, but I agree with your affirmation and visualization, "I am getting better and better at math." Then you can find out: are you getting better as you do this affirmation and visualization?

Maybe you could do affirmations and visualizations about math itself: "Math is interesting and important," or "I am liking math more and more," or something like that. Or, you could do specific affirmations and visualizations, such as, "I can do this math problem and get it right." Maybe that would help you do your math homework and tests. There are a lot of interesting experiments to do on yourself in this area, and you can see what data you get, from your homework and test scores.