I sometimes think/say a particular thing is not mentioned in Quran. Then I doubt what if it is in the Quran. So afterwards I get scared and say my shahadah that what if it is in the Quran and I've denied it. If that particular thing really is not mentioned in the Quran, would I be doing kufr since I might have added something in the Quran? Or is it an extension of my waswas?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Your question stems from whispers, nothing but whispers. So you have to endeavor to repel them and not give in to them. There is absolutely no Kufr (disbelief) in what you mentioned in the question.

Allah knows best.