Natalie, hang in there mate, you are doing a fine job. Its very frustrating being a mum because the little buggers don't include an instruction manual when they come out!!!! I'd agree with the others, check that there is no medical problem, ie colic or reflux. Or, try a paediatric chiropractor - that helped with my DS cos he wasn't the best sleeper, because his neck was out and he was in pain whilst trying to get comfortable, so needed my warmth to help soothe his pain.
Do you have a swing chair? that helped soothe mine to sleep too. Any other people around to support you and to give you a break works great too. Sometimes its fantastic to just have a break and go outside and have a nice cuppa alone with your thoughts... I always like my "me" time, makes me remember who I am, not that I'm only just a mum!!!
Whereabouts are you? Perhaps there are BB members near you that can come over for a bit and help out - even if its just to hold the baby so you can take a nap or something....
I'm just throwing around ideas because I know how difficult it is!!!!! you are doing a great job - keep your chin up hon!!