How would this person guarantee that Allah accepted his fasting and forgiven his sins?

The Prophet said that if a fast the day of Arafat the bad deeds of last year and coming year will be forgiven , then one can fast on Arafat and keep committing sins for the next year as they will be forgiven due to fasting in Arafat.

Fasting | August 31, 2017 8:17 pm
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I had the intention to keep a fast on arafah day, but now I can’t bcus of my menses, will I get the same reward without fasting? Or should I make up for it by fasting another day?
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Sheikh,I saw in one of your posts that it is forbidden to fast on 10 11 12and 13 of Dhul Hijja. But the white days (Ayyamul Beeyd) – 13th, 14th and 15th are equally recommended… Please help clarify.