How old is your LO?
lots of babies tend to eat little and often, their stomachs are still very small. Also they sometimes cluster feed... my LO used to want to feed sometimes every half an hour in evenings. It was tiring, but i just spend the evening sat on the sofa and put him to the breast as much as often.
My Lo only ever feed for 10mins max at time, sometimes even less... but he is gaining weight well. Some babies feed for a short time but feed effectively, while others feed for longer but still only get the same amount.
if expressing is working for you then carry on. But perhaps now you have managed to get your LO to latch on it might be an idea to feed more often from the breast. The more often you put her to the breast the more milk you will produce. BF is tiring in the early days, it sometimes feels like it is all you do. That you finish one round and feeding and nappy changing and start again but honestly it does get easier. By around 6 weeks you may start to notice some sort of pattern.
Make sure you eat well and drink lots of water, its very important when bf'ing. Sleep when she sleeps to.
Your not doing anything wrong, it sounds like you are doing brilliant. keep up the good work *hugs*