These are my corrections to your text:
Marcus, the main character of the book "About a boy" written by Nick Hornby, is a 12-year-old son of divorced parents. Marcus lives in London with his mother, Fiona, a music therapist aged 38 who has an unstable and chaotic love life. Marcus's father lives in Cambridge with his girlfriend. *In the first chapter of the book in which the author describes a further separation of Fiona, we notice that the teenager feels affected by the numerous failed relationships of his mother despite being used to this type of situation. Marcus is concerned and supportive towards his mother when Roger, her last boyfriend, leaves the house after an argument. We can also notice that Marcus seems to be misunderstood by the people around him, which probably is the cause of many problems in his life. However, he’s a sensitive and delicate boy. What is also quite clear is that he’s extremely observant and pays much attention to all the things that happen around him. Marcus seems to be very mature for his age, which is clear through his reaction to Fiona's break up. On the other hand, he also seems to be uncomfortable with the situation and with her unsucessful love life. It’s also important to mention that he’s a very smart and curious boy. Like most of the boys his age, Marcus has to face the same conflicts, such as fights and arguments with his mother. Despite the disagreements, Marcus admires his mother, **and shows affection and support towards her, and tries to be understanding and to accept her boyfriends. After her breakups, Marcus tries to make her feel better and to be optimistic about the situation. What caught my attention in his behavior was that even though he’s still young, he’s mature and sensitive enough to understand what is going on and tries to be careful and thoughtful.
*I've changed the order of the sentence.
**Too many "ands".