Sorry that this has happened to you. I had a badly stitched ventouse delivery too, and I was in a lot of pain. Sadly, it seems that the squeaky wheel gets the grease, even though with a small baby you are probably exhausted and you are definitely very busy. But it is worth pushing for treatment. I had a whole host of GPs (and one gynae) fob me off before I saw someone sympathetic.
From my extensive experience (and a lot of reading around on MN) I've found several options.
Surgery - this is obviously the most drastic, and some doctors don't like to restitch if there are no obvious problems as they will be replacing a scar with... another scar, and there is no guarantee that the second scar will be great. A lot of women on MN have had really good experience of surgery though (search for Fentons procedure) - it seems to be especially appropriate for women who are sewn too tight, or who have a circular band of scarring around the entrance to the vagina.
Cortisone injections - this is what I had, and they were marvellous. My consultant told me that they are only effective for 50% of women, so you would have to get lucky. There are almost no downsides to trying though, except the actual pain of injection. This is a fairly new form of treatment and not all obs or gynaes are familiar with it - it might not be a normal treatment option at your hospital.
Hormone treatment - if you're breastfeeding you almost certainly have lower oestrogen levels in your vagina - you can get hormone creams to increase the levels and some women find this helpful for healing - I certainly found an improvement when I gave up breastfeeding.
Dilation - this is very low tech and has no side effects. If you need to stretch the tissue you can use dilators (fancy medical term for dildos) to stretch and massage the tissue internally. There are also good to get over the fear of penetration (which is very understandable after having several painful attempts to have penetrative sex).
Massage - it's well worth doing massage, even if you try one of the other treatment options as it has no downsides at all. You use a simple oil like regular olive oil and massage and stretch the painful areas. I've had good success with this.
Vaginal childbirth - this is a joke, but my scar really improved after I had another vaginal birth, and my experience isn't uncommon. It's a bit of a drastic option though!
Regarding the increased size of the entrance to your vagina - this just seems to be an unspoken legacy of a vaginal birth. It's certainly come up as a topic of conversation on MN several times. Pelvic floor exercises help, but it's rare for things to look completely the same down there. I found that I hated the fact that I looked different, and focussed on it a lot. But I think that it was really because I was so upset that things didn't work properly. I was very keen to have surgery to neaten things up (which is possible), but as time goes on, I am getting less keen on the idea.
Sorry this is really long, but one last thing - it isn't too late, and you have a good GP, which I've found is half the battle. Don't forget that if you happen upon a useless gynae ("all in your head" etc.) that you are entitled to a second opinion on the NHS. Good luck.