Google translate gave me “relinquentes autem fideles permanemus” for “they left you but we remain faithful”but I know google translate can be just a tad less than accurate haha. Any help would be greatly appreciated!
Illi te reliquerunt, nos autem fideles manemus.
You can't use relinquentes, since it's a participle. The sentence you have now is "but leaving behind, we remain thoroughly loyal." The subject would be both doing the abandoning and the remaining. So you'll need to add a subject (I chose illi, "those people") and conjugate the verb appropriately.
While I chose manemus, you can still use permanemus, and there's nothing particularly wrong with it, but for some reason it's never recorded in the 1st person plural. But Livy has fideliter permaneant, so it's not a bad option.