REPETITIVE means occurring over and over but is fairly neutral. Like breathing. Or waves pounding on the shore. Or trash pickup on Tuesdays.
REPETITIOUS means the same but is often used to mean it’s tedious (makes you stress out). Like grinding one’s teeth. Or playing the same song over and over and over and it’s not a favorite song. Or that quartz clock that makes a noise every second of every minute of every day. (Did you know you can buy noiseless quartz clocks. I finally bought one for my office and its lack of noise is wonderful.)