Rhythm is not something you put together in your head each time, taking into account the duration of each note.
Even if you know that a quarter note is one beat, an eighth note is a half beat, etc., as in your textbook, you cannot play a music immediately just by looking at it.
People who can read music and play it immediately remember various combinations of notes as "rhythm patterns".
Even people who can read classical music fluently may stop for a moment when they see an unfamiliar rhythm pattern such as jazz or Latin. Therefore, people who are accustomed to more patterns can be said to have better music reading skills.
First, make a collection of rhythm patterns that you can think of in two-beat units.
Play a single note or scale in this pattern to learn the rhythm.
Once you have learned it, create a pattern that adds sixteenth notes and practice it.
A music is a combination of pitch and rhythm pattern, as you look at various musics and get used to them, reading music will no longer be a struggle.
At first, it is a good idea to practice familiar songs on an instrument while looking at the music.
Students will naturally come to understand the relationship between the music and the sound by seeing how a familiar melody sounds when written on sheet music.