first step to filing a grievance is finding out whether your confidentiality was in fact violated...

example here in NY signing a confidentiality form means you are giving the therapist permission to talk with parents or other treatment providers...

you also say you still live with your parents. if this is because of mental problems for which your parents are your legal guardians then your confidentiality was not violated, that is if you are in the USA. the legal guardian of a mentally ill adult has the right to know what is going on with that person for which they are legally obligated to make mental and physicla health decisions...

also some mental disorders contain the symptom of paranoia... not saying you are but just saying this may be a factor in a legal issue such as a violation of confidentiality, as in if your mental disorders and meds have a past history of making you believe things like others are out to get you, others are conspiring against you, others are breaching your rights, this will come up in present legal matters of the same problem

my suggestion is talk with your parents and treatment providers, find out why they needed to discuss you and your problems and whether this is something that actually violated your confidentiality.

as for the opening your mail thats easily fixed. you are an adult you can go to your local post office and obtain a mail box. then you go to the post office and get your mail out of your mailbox with a key. only those you give your key to can get into your mailbox at the post office.