The reason why you can't concentrate is simple. What the teacher said can't be linked with your existing knowledge.

That is what the teacher said, you can understand very little.

The solution is simple,

The first is preview before class. Look at what the teacher wants to say in advance, and do a problem, so you must use your own brains.

Even if you can't, because your brain has gone through the process of thinking, it will print the preview in your mind.

It's much easier to accept lectures in class.

The second is to find ways to get interested in mathematics. It is suggested that some interesting things about mathematicians can be searched online.

For example, when learning trigonometric functions and the limit of sequence, search the people who put forward these concepts and see their times.

What happened around them or something.