Rex lost everything. He lost every single one of his brothers, his Jedi, the Republic that he had been proud to serve.
His brothers are either dead or suffering a fate almost worse than death. Their free will taken away and being forced to fight for something they no longer believe in.
General Anakin Skywalker is presumed dead. Either murdered by the clones he was serving alongside or murdered by a Sith lord on Mustafar.
The Republic has fallen and the Jedi along with it.
The only thing Rex has left in the end is Ahsoka.