I am truly confused because I have broken an oath with Allah. My Nadhr with Allah was to refrain from a secret haraam act and never commit it again, and if I did I must pay Kaffarah and part of my will I wrote that I must recite 1 million salawat, I just don’t know what to do or how I’m going to recite 1 million times the salawat. Please show me if there is another way I can solve this out. Can I just pay the kafarah or do I have to fulfill it?

Unfortunately you have to fulfil your nadhr. You can do it slowly for example everyday few thousand salwat