Some people are just too curious and/or nosy for their own good, cannot stand to feel left out or like someone may be talking about them. It is never right to read anything another has written someone else without their consent. It cannot further/help a relationship.

I hope you have discussed this breach of privacy/personal space with your partner and gotten her to understand she cannot do anything like this again or you will have to leave. What your partner thinks of therapy is her opinion for her use and is not about you and what you are doing in therapy.

I do not understand why you feel you cannot discuss this issue with T; it doesn't really matter if it is your partner, your parent, your child, your lover or your friend who has invaded your personal space; there is a boundary there that you need to make clear and that you should not allow to be crossed. Find your anger and stick with it until it is resolved; there is no defense or excuse. It is like people who steal because "it is there". Spying and invading another person's privacy is not grown up or acceptable in a relationship.