Hi Bobbert.
Concerning reciprocal statements like "el uno al otro", "unos a otros", "el uno del otro", etc., I have always heard these in masculine if you're talking about men or mixed genders. If you're talking about only women, I've heard "la una a la otra" or "unas a otras" etc.
I've never heard it mixed though, like "unos a otras", and I don't think that sounds right.
Now onto the translation...
"We have known each other for a long time and trust each other, but she still never confides anything in me."
How about one of these options?
"Tenemos mucho tiempo de conocernos y mucha confianza, y aun así, ..."
"... no me comparte sus secretos"
"... no tiene la confianza para decirme todo"
"... no se abre del todo conmigo"
"... no me cuenta todo"
"... no me confía nada" (this option that you had can be used like this)
Maybe a native Spanish speaker can help too. In this case, "confide" in English means "to share things that you would not tell anyone else."