Hi Rebs - congratulations on your new baby. Sounds like you're doing a great job if you are managing to breastfeed and look after a 6yr old.
I'm no expert on these things but I think as 2nd time mums we put extra pressure on ourselves to make everything perfect - but it won't be all the time - I'm sure your eldest understands to a certain extent - does (s)he have any other relations or family friends to spend time with (to get a bit more attention and to give you a rest)? Also don't be too worried if you have to put baby down for short periods to attend to the other - a little bit of crying shouldn't do any harm
Other than that I'd think about contacting your local childrens centre - they can talk to you and point you in the right direction if you need further support.
There are loads of mother & baby groups around too - where are you based?
Best of luck - do speak to your GP if you don't start to feel better x