Forever cluster feeding

LO is 5 weeks old and I swear she has cluster fed every single day of her life. I had to suppliment with formula starting week 2 because she wasn't gaining enough weight. She only gets 2-4oz of formula a day and I am trying to build my milk supply but goodness this is hard. She doesn't nap much, always wants to eat...I don't pump after she eats because she wants to eat again within an hour, and in the evenings she gets so frustrated that she yanks on my nipples and they hurt so badly. Does this ever get better? :( I end up giving in and giving her the formula in the evenings because the pain and exhaustion become too much, but am I hindering my supply?