If you are a U.S. conditional resident who obtained that status through marriage to a U.S. citizen, but you are now in the midst of divorce proceedings, you haven't necessarily lost your right to a U.S. green card (lawful permanent residence).
However, as discussed below, your success will depend on the timing of your divorce, the date upon which your conditional U.S. residency expires, and whether you can prove to U.S. Citizenship and Immigration Services (USCIS) that your marriage was the real thing, or bona fide, in the first place.
(See Divorce Issues for a U.S. Immigrant or Permanent Resident for information on the broader immigration issues caused by divorce.)
Conditional residency expires two years after its approval. Check your green card for that exact date, and keep track of it carefully.
Within the 90 days before that expiration date, all conditional residents who intend to stay in the U.S. permanently must prepare and submit USCIS Form I-751, asking to convert to permanent residence.
Being in the middle of a divorce is not an excuse for submitting Form I-751 late. Once you have passed the expiration date, you're out of status (meaning have no permission to remain in the U.S.), and USCIS will place you into immigration court proceedings. This is likely to lead to your removal (deportation) from the United States.
Oddly enough, once you've started divorce proceedings, the ideal situation is for the divorce to be finalized before you have to submit your Form I-751. With a final divorce, you can submit Form I-751 without the help of your U.S. citizen spouse, by asking for a waiver of the usual joint filing requirement and proving that, while your marriage was bona fide, it was ended by divorce, but you still wish to pursue a U.S. green card.
If your divorce will NOT be final before you submit your Form I-751 to USCIS, your situation gets complicated. You can't apply for the divorce waiver, so what can you do? Ideally, under this circumstance, your spouse will still be willing to file the I-751 jointly with you, and help supply documents showing a bona fide marriage, which is the "normal" way this form is filed, reaffirming that your marriage was valid.
But what if your U.S. citizen spouse isn't willing to file jointly with you? You might, depending on circumstances, qualify for one of the other waivers of the joint filing requirement; perhaps one in which you show that your marriage was entered into in good faith but you were battered or subjected to extreme cruelty by your U.S. citizen spouse; or one showing that termination of your permanent residency and removal from the United States would result in extreme hardship.
Again, to qualify for a waiver of the I-751 joint filing requirement, you'll need to show proof of a final divorce, and that your marriage to the U.S. citizen was legitimate in the first place.
All of this will require attaching copies of documents to your Form I-751; not only the divorce decree, but evidence of your bona fide marriage (collected AFTER your initial approval for residence), such as children's birth certificates, affidavits from marriage counselors who tried to help you patch things up, financial records showing that you shared bank accounts and debts, joint leases or mortgages, and so on.
Dealing with a divorce when one is a U.S. conditional resident is complicated, and you'd be wise to retain an experienced U.S. immigration attorney. The attorney will assist in preparing and filing the I-751 form and suggest convincing evidence to prove that your marriage was bona fide; or will help you apply for a different waiver if the divorce doesn't come through in time.