I think noblegiraffe has made a great point. Would you carry on if it was more comfortable? If so, call in as much professional support as you can. This is your HV's job! Ask them what is available.
Don't worry about volume when expressing. What always amazes me is how little I get in the first 10-20 minutes, but then how it starts to spurt out. It can be a slow process.
Three weeks us a really tough stage. The fatigue us catching up with you, the enormities of what you've let yourselves in for has hit home, and the reality of breastfeeding on demand (ie all day done days!) is upon you. This doesn't last forever!
If you fear nipple confusion, try making sure she's taking enough boob. Nipple feeding is painful! If you google exaggerated latch or flipple technique you'll see how to get more boob in. DD still does a bit if cheek sucking rather than jaw moving during her first feed after having a bottle. Don't be scared to delatched and start again.
I found bf got easier at 3/4 weeks, and then at 6 weeks the fog lifted and everything suddenly seemed much easier. I now love that I can just whip a boob out anywhere and feed without worrying about washing and sterilising, or how many ounces she's taken.
DD is now 11 weeks, ebf (bar a couple of bottles of formula yesterday when I snuck out for a massage and swim!) and sleeping through. If you want to continue, it can be so easy in just a few weeks.
All that said (and sorry for the essay!), continuing or stopping is entirely your choice. I remember your previous thread. You've done brilliantly.