Sometimes, such question have much more meaningful interest than just exercises. The process of learning mathematics, studying in the right way and getting to properly understand the true meaning of them is something that not only bothers lower-level or standard level students (of any age) but also anyone who is involved with mathematics. The process of learning never ends !
Now, after my philosophical entry, let's get to the facts. An adult may find it harder to adapt to studying mathematics and understanding new ideas compared to its younger self, because a younger brain is keen on learning things easier, while being fresher and more relieved of obligations. But, it is never late to learn new things !
Getting down to studying now, it really depends on the level of mathematics you want to comprehend. It also depends on the subject you're trying to study. For example, if you are interested in Geometry, visual representations, drawing sketches, seeing thorough examples will be of very big help. On the other hand, a more algebraic-theoritic or applied related mathematics course, needs first of all very good understanding of the theory and the ideas behind the tools used. What defines a good book for every student, while primarily opinion based, would always come down to the fact that it boasts a healthy balance of both theory and applications, while also some real life examples.
Understanding (fully and correctly) the theory behind the tools and the ideas of every chapter is very, very important. True understanding and grasping of things as mathematical substances is a really strong weapon into developing a strong and rigorous mathematical thought and approach to problems. I would suggest that your first priority should be the proper understanding of the theory, regardless the subject, before moving on to studying examples and exercises.
Important : A lot of students and studiers, make one very common but big mistake. Studying solved exercises by just reading through them and watching them does not make you truly better in mathematics. You may grasp some stuff, but the most important thing is practice. They say that practice makes perfect and while no-one can be perfect in mathematics, it definitely makes up for a lot of struggles. At start, it may seem a grind or it may make you feel like you have gaps in your knowledge, but continuous practicing (which means picking up a pen and trying to solve exercises) is very important. Even if you cannot solve an exercise completely, the process of thinking and trying also makes up for a very good level of future understanding.
Finally, for visualising concepts behind mathematics but also getting to see more stuff, internet is your friend. You can find a lot of applications, videos, textbooks of any kind and even online courses (most of them free).
A note : Do not get frustrated and never feel sad of not understanding something. Everybody goes through the process of failing to solve something or understanding something, but we shall never give up on our attempts to make ourselves better and try to find solutions to our problems. Seeking help by a professional mathematician, buying books or studying yours, searching online or practicing by yourself until you can solve the exercises that bother you, will lead you to getting rid of all the initial frustration.
A note (2): If any more information about why you want to learn mathematics or what courses and subject exactly you want to learn, are very welcome and you may carry on by commenting !