Is it necessary to make up for days of fasting where the intention to fast was doubtful due to intrusive thoughts about the intention?

Do not pay attention to whispers [waswas]. The intention for fasting is easy; it is simply that it crosses your mind that you will be fasting tomorrow. A Muslim usually does not neglect the intention for obligatory fasting. Some scholars are of the view that a single intention suffices for the beginning of the month, and you may adopt this opinion if you are afflicted with waswas. Do not make up any days after Ramadan (definition) due to doubt about the intention, because this doubt is merely waswas, and making up the fast at that time would be giving in to waswas. The cure for waswas is to ignore it and turn away from it.