This is the definition:
‘personal data’ means any information relating to an identified or identifiable natural person (‘data subject’); an identifiable natural person is one who can be identified, directly or indirectly, in particular by reference to an identifier such as a name, an identification number, location data, an online identifier or to one or more factors specific to the physical, physiological, genetic, mental, economic, cultural or social identity of that natural person;
You seem to mostly understand the definition.
I have a youtube account and create a comment, is this comment now considered my personal data?
It is personal data about you. It isn’t necessary “your” data though; it is controlled by YouTube. Being personal data gives the data subject certain rights and creates obligations on the controller of the data but it doesn’t change who controls the data.
If that is true, then wouldn't any data that links to user in database considered his personal data?
If the user is a data subject; yes. For example, data about corporations or governments or your cat or the majestic African wildebeest is not personal data unless it can be linked to a data subject - like if it can be worked out that the cat is your cat.