Adding to Tomísimo's explanation:
The collocation is "confiar en"; so we'd say: "confiamos uno en el otro" ("unos en los otros" in plural). This is the general formula when both people are masculine, and frequently between a man and a woman.
However, as there is more and more gender visibility, you may find more often:
- Confiamos (la) una en el otro (female - male)
- ... (el) uno en la otra (male - female)
And always, between feminine subjects: "confiamos (la) una en la otra"
As for:
Nos conocemos desde hace mucho tiempo y confiamos (el) uno en el otro, pero todavía no me confía nada.
-> The first "el" is not wrong, but it feels long, so we frequently omit it.
-> "Nos fiamos uno del otro" is an interesting construction. I would never use "fiarse de alguien" in a close relationship.
This verb is mostly used in negative sentences, and if I were to use it in a positive sentence, it would mean I don't have obvious reasons to mistrust someone, but I don't know them enough as to actually trust them with something important.
- ¿Ese amigo tuyo es de fiar? -> I don't know your friend, but I want to know he's honest enough.
- No me fío de los dueños de casinos. -> I have some reason to believe casino owners can't be trusted.
- A los clientes no se les engaña, porque se fían de uno. -> Clients don't have a close relationship with you, but you know they don't expect to be swindled by you.
The second part of the sentence:
- no confía nada en mí -> This contradicts the first part of the sentence. Either she trusts you or not. So this is not the correct collocation to imply her telling you important things.
- no confía nada a mí
-> This construction doesn't make much sense. You may say "no me confía nada a mí", but this changes the meaning of the sentence. It would mean she doesn't let you take care of important business. If there weren't a context, "aún no me confía nada", could be interpreted this way too.
And only for reasons of style, we normally avoid repeating verbs in a sentence, so we prefer to change something. Unless you're repeating the verb on purpose, Tomísimo's alternatives are great.
