Hi, just thought I'd introduce myself. I'm living with my guy and his ex of 19 years was recently diagnosed with BPD, and is probably undiagnosed with other disorders. We have 5 kids between the two of us - my kids are 18 and 15 and his kids are 18, 17, and 11. We've been together 2 years and it hasn't been easy. I'm grateful for the diagnosis - my guy's oldest told us about it - because I feel like it'll give us a point of reference. I've been doing some research for my own understanding - a lot of things make more sense now - and I was really glad to come across this forum. I have already picked up on some really good tips for my own health and wellbeing... Any advice is welcome.

Welcome! I just found this site about a year ago, and the people here are awesome. So knowledgeable and helpful, and there's a sense of unity in knowing that so many other people on here have gone through something similar to whatever you're going through at that moment. You've come to the right place.
I was introduced to the book "Stop Walking on Eggshells" a little over a year ago, and that book was extremely helpful. It helps you understand what BPD is and isn't, that you aren't alone, how people develop BPD, and tools you can use in your interactions with someone who has BPD. IMO, reading that book is the best place to start so you can understand what your partner and his kids have been - and still are - going through.