Sorry to hear you are struggling. I might need a little bit more background in order to give any useful input. Is it that Tom and Sara are the more established couple? In what circumstances did your relationship begin? How long have you been together? What are your living circumstances? Is Sara the only one working?
My triady situation is very new but everyone is very committed to making it work. "My" couple
has sought out a third for years, so that creates a different set of pressures than if you stumble upon a triad situation accidentally. Still, I think the strain that is caused by one couple/person in the triad wanting to progress faster than the others is pretty universal. It is not a sign of doom to struggle with this, IMHO, if Sara is willing to try. Is there any history between you and Tom that would explain Sara's insecurities?