How can I not forget university mathematics?
In order to learn anything you need to cause physical changes in the brain. This requires repetition and reinforcement. The brain actually re-wires itself by forming new connections between neurons. Your initial reading is probably doing little for you as you just pass over the material. If you read, for example, The Iliad, it is a nice story, but you won't remember much of the detail of it after you finish. Only a few things will stand out. And mathematics is worse because it is the connections between things that are essential. 

Your second pass, doing all the exercises is better, but apparently not enough. 

Engage with the material. 

As you study take notes as you go. Summarize each chapter of the book. Write down the three most important ideas in each chapter. Better yet, each time you have a "study session" write down what you think are the most important ideas (but not everything). When you finish, go over those notes again and write down the three most important ideas. 

When you go back to study for another session. Try to remember what the important ideas were from the previous session. If you can't go back and review your notes. 

Periodically, review your notes. You don't have anyone to give you an exam, which is the trick professors use to get you to review, so you will have to make that part of your regular process. 

If you take notes on index cards you can carry your summary cards with you for review. If you have to wait for anything for a few minutes, you can pull out your cards and shuffle through them. You can also rearrange them and create mini-decks of important ideas. 

But the idea is to force the important concepts from short term memory into long term. Memorization isn't the goal, however. It is making the remembered ideas useful. Connecting one idea to another. A math problem seldom depends on only a single idea for solution. You need to combine them. Memorization alone won't do that for you, but solving lots of problems will. There used to be (and may still be) books you could buy that had nothing in them but exercises. I once used those books to help solidify my knowledge. 

You can also make up problems for yourself. If you want to know what, for example, derivatives can tell you about the shape of a curve/function of a certain kind, graph a lot of such functions by hand, on paper. If you graph, say, 100 rational functions by hand using the first and second derivatives, you will get a feel both for rational functions and for derivatives. 

It is that sort of reinforcement that leads to learning.