Obviously I can't answer as to whether you should stop breastfeeding or not, but just to let you know you are definitely not alone when it comes to having no luck with expressing! I tried the AVENT manual pump and the Medela electric pump, both were good at the start then after about 1-2oz it would slow to a drip. On a good day, and these were few and far between, and after several long bouts of expressing, I would end up with one 6oz bottle! Usually I might get 2oz or so, which was fine when she was wee but obviously as her feeds increased I could not keep up (breast or bottle) and eventually switched to formula.
Anyway, like I said, I can't help about knowing whether you should stop or not (you know only you can answer that), but don't feel alone because when I posted about the problems I was having it seems very common with expressing and something I wasn't aware of prior to breastfeeding, I thought it would be easy expressing and it isn't! Don't feel pressured into continuing breastfeeding through guilt, but, as you probably know, you probably need to decide soon as if you decide to go with formula then you will need to start weaning Isobel.
Goodsoulette always has excellent advice re breastfeeding so keep an eye for her and point her in this question's direction :-)
P.S. I am now laughing hysterically as before I postedmy answer I went through to to check for typos and realised I had been using the (Nelson) Mandela pump! PMSL