What is the ruling on doubting the intention during prayer due to satanic whispers, and what is the effect of being preoccupied with worldly matters such as talking and looking before prayer on the validity of the intention?

You have reached a significant level of whisperings (waswas), and its cure lies in turning away from it and not paying attention to it. When praying, bring to mind its intention and start directly. Ignore the devil's whisperings that you intended another prayer or changed your intention. The intention is sufficient a short time before the act of worship, and some scholars have even allowed it a long time before, as long as it is not diverted to something else. So, do not give in to the whisperings and ignore them completely.