How can one overcome the obsessions and doubts about the validity of the Islamic religion and choosing Islam as a religion, while feeling guilty and fearing disbelief (kufr)?

You are a Muslim as long as you detest these whisperings (waswas), feel aversion towards them, and experience guilt because of them. Therefore, ignore them and pay no attention to them. You will be rewarded for fighting them. The proof of the soundness of Islam is the Qur’an, with which Allah challenged mankind and jinn.