Is what the questioner does—talking and making phone calls with a girl, followed by actions like those of married people—considered zina (fornication/adultery)? And what is the way to sincere repentance from that?

This act, even if it is not fornication (zina) necessitating the prescribed punishment (hadd), is one of its مقدمات (precursors) and دواعيه (inducements). Therefore, you must repent sincerely and completely sever your relationship with that girl. Among the things that will help you in this are: being mindful that Allah is observing you, frequently contemplating death and the horrors of the Day of Judgment, occupying your time with the remembrance of Allah and acts of worship, befriending righteous people, increasing your reading and listening to admonitions, and seeking help from Allah the Almighty and striving in supplication to Him.