Am I obligated to correct the divorce decree to prove that I am a previously married woman after my ex-husband secluded himself with me and had anal intercourse with me, then divorced me and claimed he had not secluded himself with me?

There is no need to change the divorce certificate, and no need to fear deceiving the suitor. Since your virginity is still intact, you are a virgin and not a previously married woman (thayyib), because a virgin is one whose front passage has not been penetrated. Anal intercourse does not affect virginity, so do not go to court, and repent to Allah Almighty for what occurred between you and your husband regarding the forbidden intercourse.