i understand how you feel. Maybe you could buy some food you like and enjoy it or watch a show you like or buy yourself some nice clothes that make you happy. Or if there are hobbies you enjoy try doing those. Or get some nice smelling soaps or something - the body smelling wrong annoys me. Little things make a big difference IMO.
About the girlfriend thing, sorta opposite for me as im a lesbian and the headmates usually date men. don't know how your system works, but could you still pursue relationships that make you happy? Like when some here were dating a guy, i was dating a girl at the same time. My girlfriend and their boyfriend knew about it so it wasn't dishonest or anything.

there were rules and stuff for me but it made me so happy to have someone that i really cherished. It gave me a sense a normal. Maybe if youre accepting of their relationship they'll let you have one too? It's worked for us but then again we're poly so that is how it goes and we're all in agreement.
i bet youre really overwhelmed! try to find some stuff for yourself that you enjoy on the outside so you don't feel so left out. And maybe you can be friends with their SO. Love, trust and compassion are good things for any sort of relationship; like friendship or something more romantic. But you are totally entitled to confusion and frustration, take things at your own pace as you get settled into life again.
And you can always come here to talk!