Tania,
Please seek some support and advice regarding the breastfeeding as you have said it is something you really want/wanted to do. I was topping-up my DS with formula, and I am so relieved that I could manage to give that up as it's twice the effort to have to do both (bf and ff). Your DS is still so young - his tummy is still tiny. Little and often might be better. The volume of formula 'suggested' is very different to the volume of breastmilk that your DS might need.
At least if you can get some proper advice, you can feel more at ease that whatever decision you make is the right one for you both - not something you felt forced into.

Take care of yourself - it's a tough gig.