If I was married to a US citizen for 5 years but never applied for green card, is there anything that I can do now that I am divorced?

Unfortunately no. Unless you get remarried.

Unfortunately, all you can now do is wish you had done it while you were married.  You are not now eligible for a green card on the basis of marriage since you are no longer married.  If you happen to get married again, to a US citizen, and if you are eligible for adjustment of status (if you entered the US lawfully but overstayed), you will be able to apply for adjustment of status to permanent residence once you are once again in a valid, bona fide marital relationship.