Is it obligatory to make up the days that I previously missed due to menstruation that lasted more than 15 days, or is only expiation required, knowing that fasting conditions are currently difficult due to studying abroad?

The questioner is obligated to make up the days she missed in Ramadan (definition) as long as she is able to fast, because feeding is a substitute for fasting only for those who are unable to fast, an inability that is not expected to cease. If the delay in making up the fast was due to a valid excuse, then she is only required to make up the fast. However, if it was without a valid excuse, then she must make up the fast along with feeding one poor person for each day, unless she was unaware of the prohibition of delaying, in which case the expiation (fidyah (definition)) is waived.