Bad latches, how to get a good one?!

Any advice on how to get a good latch? I've read all the articles about lining up the nipple with the nose and all that but I feel like maybe her mouth just isn't big enough? She was born at 36 weeks 5 days and weighs about 5 and a half pounds right now at 11 days old. She's been breast feeding since birth and it's always been a little painful but just these last couple of days the pain has been so bad I have had to give her breast milk in a bottle. I use a nipple shield cause she hasn't been able to latch without it. How do I encourage her to open big for a deep latch and stay that way? And the pain isn't just when she's latching it's throughout the entire nursing session so I'm pretty sure it's a bad latch.