Telling your boss about mental health problems

This is a bit of an ironic post for me to make, because guess who hasn't told their new boss about their mental health issues yet? *Waves hand frantically*. But, as I have some (upsettingly) obvious self harm scars, I imagine it's only a matter of time before I get asked, or *hopefully* my boss has chosen the tactful route, noted my scars and realised that there may be some underlying issues.
Before, this job however, I have told almost all of my past bosses about my MH issues. When I worked as a waitress my 'off days' made a massive difference in my ability to do my job. I was a little more jittery when speaking to customers, and a lot less capable of handling angry chefs. It was necessary to talk about it, and again, my scars were hella obvious. 
I've only been in my new job for around six weeks, and if I'm honest, I'm really scared to open up this time. This is the first job I've had that I've genuinely cared about losing. I'd be absolutely heartbroken if my mental health issues managed to damage the chances I have there. And no, my workplace isn't a scary place to be, or a hostile atmosphere. I believe that they'd probably put things in place to help me if things got rough, but it's that niggling doubt that's holding me back.
Until I get my footing secure in this job, I likely won't speak out unless I have to, and I know this 'sets a bad example' and all that jazz, but it's the truth. However, in my past jobs things have always gone okay. It can be hard to talk about things in person, so I've spoken to managers via email in the past to explain that I have mental health issues. I tend to do it as a security net; if I have a down day, or if I'm struggling I can then tell them that my mental health is bad without having to explain that I suffer with mental health issues in the first place.

Part of me thinks that it's none of their business. If I do my work, then I shouldn't have to mention it any more than I should have to mention a physical health problem. But, telling your boss can allow them to help you to do your job more successfully. In my past job, my manager would ask me how I was doing on a certain week if she was going to give me more than my normal six shifts. She knew that having too many shifts in a week could make me go downhill, and her knowing that took the pressure off. It gave me the ability to say no to more than I was capable of, and meant that someone had my back when it came to making sure that my needs were met.

What I'm trying to say is do what you think is best. You don't *have* to tell your boss about your MH problems, but equally it can be really useful to do so. 
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