When I was 17, I decided I was going to be a nurse. At the time all you needed was to have 5 C-grades at GCSE. As such I stopped applying myself in sixth form once I discovered I'd got in. I walked out with an E at AS Maths, an E in A-Level Biology, and a C in A-Level General Studies.When I started Nursing college, I didn't bother applying myself, and due to circumstances that were for the most part, my fault, I ended up leaving. I got a job in a nursing home and got into somewhat of an emotionally abusive relationship. I then got a job in a coffee shop, and eventually had the strength and common sense to escape the relationship. I moved out of home and proceeded to not do much with my life other than work the coffee shop. I got another girlfriend, this one actually wonderful, who convinced me to try education again. I enrolled in Night class one night a week, and two years later, to my own surprise, I recieved a B in A-Level English. I by sheer chance enrolled on a three month counselling course, and once again to my surprise, it fit me. I emailed a local university for some information on the course, and maintained semi-regular contact with who I later discovered was the course leader. I got an unconditional acceptance.
I am now 28 years old and have my next few years planned out. I'm just finishing my second year at university, and whilst poor, I am comparatively happy. Its hard work, (especially this weekend), but its going to pay off.
It worked out for me. I don't know how or why, but it did. Its taken me ten years to get where I am now, and it still scares the hell out of me. I pulled my thumb out and applied myself, and each time, I surprise myself at what I'm capable of.
You can do this too. I make no lies about it being easy. I lived in a nice area, both parents, didn't need for much, and only had myself to blame. I haven't had anything like the upbringing you say you've had. This might make it easier, this might make it harder, but I believe you can do SOMETHING. I'm not going to make any promises that you can become a Doctor-I know little about the grades but given that it seems to do the most basic courses you need high grades, you HAVE to apply yourself.
I don't know if resitting your GCSEs is a good idea or not. Hopefully someone else will see this and have a better idea, but apply yourself. becoming a doctor takes seven years, and it takes seven years for a damn good reason. Application of yourself is the key. Cut out the dross from your life. Are your friends really your friends, or are they just trouble waiting to happen. Establish what is crap in your life, and cut as much of it out as possible. I think its good that you recognise that you don't have an excuse to be lazy.
Whatever you're going to be in life: Good luck. The world is a harsh place and its hard to find a niche. I'm not saying you can become a Doctor-but you CAN do SOMETHING.