Right - that's good - but what if they tell bosses? My HR person would tell me everything...
The HR person might say they won't tell, but.... they do. Even though it's against the law to discriminate against disabilities, it's very hard to prove. It could keep you from getting a raise or an advanced position - if they think you're not going to be reliable, even though in the future you would be the best person for a higher position. I suggest to protect your privacy as much as possible regarding your mental health - would be better to say you have an ill family member you need to take time off to care for (almost the truth-the family member is you!).
Unless you have an awesome boss who you can tell anything to?