Must one who doubts his intention to fast due to obsessive thoughts and swallowing mucus repeat the fast, and is it permissible to adopt the most lenient opinion regarding this?

The intention for fasting is achieved simply by the thought occurring to the fasting person at night that he will fast tomorrow. Uttering it verbally is not prescribed. The pre-dawn meal (suhoor) is an indication of the existence of the intention. Swallowing mucus does not invalidate the fast according to some scholars. As for hesitation in breaking the fast, it does not invalidate it. However, if one resolves to make up the fast while rejecting the intention to fast, then the fast is invalidated and must be made up. There are also other things that invalidate the fast besides eating and drinking.