Ok in the light of no one else answering yet I've come back 
I had this too despite vaginal births. I still have a little gap and the hernia hasn't gone away, though I wonder if it would with the right physio or exercises. I haven't had time yet as I live on my own too and ds3 is still a toddler.
I've been told I can have surgery to fix the hernia, which is only small, but I am putting it off as you can't lift for a while afterwards and I don't have any help either - toddlers are a lot heavier than babies too - so I am waiting till he is big enough not to need lifting and then I might go for it.
My strength has been Ok - I was very weak after he was born but it has gradually restored itself - I believe walking rather than driving will get it back quickest, my stomach was far better sooner after my other two than this one, and I didn't drive so much then, and walked more.
So don't give up hope. You will feel like a wreck for a little while, it's normal but it DOES improve massively - my stomach is nearly flat now, and getting flatter slowly all the time, which I didn't think it would this time.
Please ignore your GP's advice - doing sit ups is a very bad idea when you have a separation, unless it is very small indeed. You need core strength and as far as I know, pilates, Yoga, certain other exercises and so on can really help with this.
I'm not sure how you go about getting this sorted out, your GP might agree to refer you for physio, otherwise it can be had privately but at a cost.
Sorry not to be more helpful. I hope you get some more advice from more knowledgeable people soon x