Can marry her and petition her if I got my citizenship through my previous marriage?

Generally, a U.S. citizen may marry a foreign national of the same sex and sponsor her in a marriage-based adjustment of status application, and this is true regardless of whether the U.S. citizen previously had been married to an opposite-sex spouse and had obtained Permanent Resident status based upon that first marriage. In that application process the U.S. citizen and new spouse not only would need to show that they are living together in a bona fide marriage, but the U.S. citizen spouse may need to show that her previous opposite-sex marriage was bona fide too. USCIS adjudicating officers have received some training relating to this, but they are inconsistent in the level of understanding sexuality and the issues facing non-conventional marriage history. It would be wise for you and your fianc?e to consult with an immigration attorney and especially one experienced in addressing these issues - who after learning all of the relevant information could advise about immigration eligibilities, options and strategies, and who could then offer legal representation in the application process.