I’m reading up thru articles myself but they might miss things out. What can I do to make my friend feel happier? 

How do you comfort them better and is there anything, as a BPD person, would like your friend to do? Like show more that you care about them? Like how do you be a better friend to someone with BPD

—The following is lowkey irrelevant maybe—

I have my own mental health issues as well which led me to isolate myself from everyone for 5 months including my BPD friend, until I started talking to her a couple days ago again. I don’t think I can get out of “my cave” just yet, but am I hurting her by isolating and does anyone have a suggestion for a compromise if what I do does hurt. I could ask her but I’m still too “socially anxious” to (whatever the term is).
Also I have been replying days late to her messages, does that also hurt her