Very sore nipples

After a bit of advice. My little boy is 4 weeks old now and he's been mainly formula fed and I've been expressing to give him some from me but it's never a lot. I've had breast feeding support round to my house 3 times and been to a lactation consultant but today I'm in so much pain after having him feed all day yesterday on me. I could barely sleep. Used a shield today and it's filling with blood and so painful. When do you just give up?