Q: My spouse and I divorced before it was time to file the I-751. Is my green card case automatically over?
A: No. A conditional resident can ask to waive the joint-filing requirement if the marriage was entered into in good faith but later ended in divorce. The key issue becomes proving that the marriage was real when you entered it, not whether the marriage ultimately lasted.
Source: USCIS Policy Manual, Vol. 6, Part I, Ch. 5: https://www.uscis.gov/policy-manual/volume-6-part-i-chapter-5