I understand what you are saying, but I think you are probably being a little hard on yourself. You definately need to make your supervisor aware of your medical condition, as it may or may not interfere with your PhD, but at least if it does then he will know why and i'm sure any reasonable person would not think any less of you for it. You don't need to make it a big issue, just mention it casually in conversation. If you don't say anything and then something happens it could be dangerous so i think from a Health & Safety point of view it is something that he needs to be aware of.

You poor thing. Listen, i was feeling pretty much the same last week, having also just started, but its getting slowly better. You must be reasonably intelligent otherwise they wouldn't have offered you the place originally. ( thats what i keep telling myself anyway!). These first few months are supposed to be spent reading and immersing yourself in your subject area, so you will be able to get up to speed with what you are doing. So don't panic, just take it in your stride, one day at a time, and remember, all your supervisors were once in exactly the same position as you. You have a fantastic opportunity, you just need a little faith in yourself.