Something else I thought about after writing.....
I think you are very intuitive! You seem to recognize you might be treating your friend in a "harsh" way, so I don't think you want to intentionally "hurt" her. When my BPD friend gets harsh with me, it's helpful to me when she say's she's sorry. Then, I know the "harshness" isn't about me, per say. It's how she feels inside in the moment of severe stress/anxiety/depression. She sometimes tells me, "don't take it personally." At first, I didn't know her diagnosis, so this made no sense to me. How does a person not take it "personally" when being yelled at? However, I now realize she was trying to tell me her anger wasn't directed at me personally. It was due to her own inner struggle due to BPD, anxiety and depression.
Vent all you want! I'm also fairly new to the forums, but have received lots of helpful advice here!