I made a vow to Allah Ta‟ala that if I commit a certain sin, I will fast 100 days consecutively.

Answer

It is compulsory to fast 100 consecutive days. There is no other option to compensate for the violated vow. If you fast for 99 days and miss the last day because of illness, then you have to renew the fasting from No.1. You must muster up courage and with the name of Allah Ta'ala begin the execution of the penalty. The Vow has to be compulsorily fulfilled. If you find it too difficult, then wait for the winter months when the days are short.