My dad told his girlfriend about my depression?

So I've met this person once. They haven't been dating for very long and I just recently told my parents that I've been diagnosed with depression, for quite awhile now actually. Last night I overheard my dad on the phone with his girlfriend telling her about my depression saying things like, "yeah its tough you know?" and "yeah It's hard to know what to do." Last night and this morning, I am seething with anger. This is my biggest secret (obviously i'm telling you people now but Im not ever going to meet you...hopefully ;) ) and he just decides to tell a woman that will probably be gone by next week, since he goes through girlfriends so fast (I'm 17 btw), and it even sounded like he was trying to get sympathy. I DO NOT WANT IT. I've told one friend, and only one. It makes me so angry. But I'm not the kind of person to just go up to him and tell him that I overheard him and that I'm extremely angry. What should I do?