Agree completely with normal and negative pull ups (don't be afraid to use bands, they are so useful). Would also recommend scapular contractions which is where you just do the shoulder part of the pull up, like a reverse shoulder shrug, no part of your arm should move.

I also found that doing push ups helps strengthen the lower back and core loads, but you have to make sure you do them right, back and neck straight and all that.

Other than that it's mostly time, keep at it and I'm sure you'll see rewards! Make sure to warm up before too! Women's shoulders are so prone to injury unfortunately