I don't like people "diagnosing" on the internet, but clearly something is very wrong for you at the moment. It sounds a lot like depression because when we are severely depressed we lose all sense of self and it is like stranger is inhabiting our own body - very scarey.
Have you had mental health problems in the past? How long have you been feeling this way?
IF you are mentally ill and I have no idea whether you are, then you will be one of the 1 in 4 people who are/or will suffer from depression at some time in their lives. What you need to do is to write down (in bullet points) how you feel and then if and when you see a GP you can hand/him her the list. This way you don't have to begin - it's a consultation by the way, not a chat. GPs are busy and allow 10 mins per patient. He/she will be able to tell from your symptoms whether you are depressed or not.
Can you say anything on here about your childhood (I know you are comparing yourself to your sibs and seeing yourself as a lesser mortal) this is probably because of low self esteem, but where did that come from. What kind of parenting did you have.
You say you are worried about being "judged" and "pitied" - well a GP will do neither of those things beleive me, but there is still a stigma around mental health and people do get very odd ideas about it, usually through ignorance. Anyway we are all making judgements about things all the time, so I would think this is the least of your worries.
Sorry I've asked so many questions and don't feel you have to answer them.
It realy does sound like you need to separate from your H as living with someone you hate must be very hard going, unless of course this is because of a mental illness, and if you were recovered, would you still feel like this. I suspect you would, but you can only tackle one thing at a time and would urge you to get a GP appointment and you won't be telling them anything they haven't heard hundreds of times before. One third of all GP appointments are mental health related.