Your GF has been with you for 3 years, lived with you. Talk about GF or BF, even if the 2 persons hating on each other with all their heart, living together for 3 years. They'd respect each other more than that to treat you the way your 'GF' does. YOu are obviously "needy and confused" if you come here and ask, whatever should we do to help you.
If you're a real man, you won't stand down for that bullcrap. As your GF, living with you, still treating you that way. Like you're her booty call, goes out and fux with other guys, come home and you treat her with "Respect?" Dude, move on, fux the biatch! Go find a different GF, you deserve better. If you're truly a good BF, in time when she sees you treating your new GF with all that respect, she will feel like **** about herself . She will come back crying for you, by that time. You could strongly stand up and tell her to STFU and GTFO.
If she really loves you, and cares for you. After you found out that she cheated on you, (i'll give you the EXCUSE to forgive her for cheating with THAT guy). But she still argues with you over him, and still wanting to go hang out, partying with him? She does not seem to have any respect for you, because she should know better what's going on for the fact that you don't want her to be around that guy. She cheated on you, WITH HIM!