Teeth whitening

I did my teeth whitening nearly 1 month ago and reached shade 0M1 but they didn’t last long. I can almost see them going back to same shade. I dont have any bad habits like coffee or tea or any such things. I paid a good amount of money for that. I dunno what to do now. I have been always been insecure of my yellow teeth. What do i do? I dont want porcelain veneers. I dont want my teeth to be shaved. I reached 0m1 after 2 sessions. My enamel is not that thin also. I dunno whats wrong