Please bear with me while I provide some background and then ask my question, which I hope you will be able to answer and thus aid my capacity to learn mathematics more efficiently and thoroughly.
I am 30 years old and I have always desired to learn mathematics. Until about a year ago, I have believed that I am not clever enough to grasp the foundations of the subject. In school, while I have done well in sciences, my ADHD has proved it difficult to focus for a considerable amount of time and dedicate and drill the math foundations necessary for progress onto more challenging aspects of the field. However, the desire to be able to explain the phenomena around me in mathematical terms has never left and, as mentioned, one year ago I have decided to put the effort in and started investing a considerable amount of time learning the very foundations. I am currently on Algebra 2 on Khan Academy and I am supporting the learning through other online sources. I am an autodidact.
However, my time is limited. I have other responsibilities and other fields of inquiry which demand my attention. To cut it short, in about 5 years I wish to make a transition into computational neuroscience (masters and onward to PhD.). I am going back to university next year to do my second degree in neuroscience and wish to study mathematics in parallel.
I am asking you, veteran mathematicians to provide me with advice on good habits. On which aspects of mathematics should I focus on past Algebra 2. Which calculus should I learn? What textbooks do you recommend? Any online lectures? Am I still able to grasp advanced mathematics at 30? How much time, on average per day, should I dedicate in order to make progress as efficient as possible. What habits should I culture? I normally do 2-3 hours per day at the moment but I find that it is not enough; the progression is steady but lacks thorough understanding that is based on underlying concepts which would make my knowledge of mathematics less superficial.
Any advice which you can give me will be deeply appreciated, considered and applied.