What is the reason for doubts in creed and faith, feeling the death of the heart and being unaffected by the Quran, and the fear of deeds not being accepted and of ostentation, despite adherence to prayer and fasting?

Congratulations on your commitment to prayer. What you have described is a satanic whisper (waswasah), so ignore it and occupy yourself with learning and inviting to Allah, associate with righteous people, and minimize being alone. Allah has forgiven the whisperings of the self as long as they are not acted upon or spoken about, as stated in the Hadith: "Indeed, Allah has forgiven for my Ummah whatever their souls whisper to them or speak about, as long as they do not act upon it or speak of it." Therefore, what you have mentioned does not constitute disbelief (kufr).