Chloebruce - do you have a National Childbirth Trust or a La Leche League near you? If you do, they will probably have a breastfeeding counsellor, who will be more than happy to give you support and information. The NCT may well have Post-natal supporters, and Bumps and Babes groups, which are great sources of support and friendship.
I ended up formula feeding all three of my dses - I had problems with my supply. All three are now strapping, healthy lads, doing well at school or university, and having great fun in their lives.
Breast feeding is an amazing thing to do for your child, and just the fact that you have managed to keep going for so long is wonderful, and you should be proud of yourself, not guilty at all! But there are many other amazing things you can do for your child as he grows up - I decided that making my own purees when I was weaning the boys was one thing I could do, even though I couldn't bf, and I carried on cooking mostly from scratch. But that is only one thing - there are so many others - playing with your child, talking to them, listening to them, reading to them, cuddling them (even when they are great big teenage boys, they still want cuddles with their mum - though maybe not in public
), supporting them at school, and in the other things they want to do - the list is endless.
What is very clear to me is how much you love your ds, and how committed you are to being the best mum you can be. Just remember, you don't have to be perfect - none of us are - and when you love your baby as much as you clearly do, the rest will flow from that.