Employer forcing to serve notice period

I am a second year graduation student and was hired by a firm as an intern for six months. After two months of my internship, they offered me a two year fellowship. The offer letter said that I will be paid on prorata basis. According to the offer letter I was supposed to work for 8 hours a day for six days a week. These eight hours do not include any breaks. The offer letter also said that if I leave the fellowship in between then it will forfeit all experience certificate, certificate of completion and any references. The offer letter also said that I will have to serve a notice period of 3 months. After completing 15 months, my health deteriorated. Thus after discussing with my family I decided to quit the internship at the cost of forfeiting all my internship certificates. My last day of working is in September but my boss has said that I will need to extend it as I am lagging by the number of hours. I told him that since I am only being paid for the number of hours I logged in, then why should I complete the hours. Especially when the in my offer letter it was just mentioned that I need to complete 3 months of notice period and nothing related to the hours was given. My health is deteriorating day by day and working for longer number of hours only triggers my migraines. Also I have to appear for a national level competitive exam by the end of this year hence I cannot extend my last day of working Please let me know what should I do in this situation. Ps. ~ Also, as this was during COVID, I signed up cor working remotely. Theh would track my work using a software.