Can anyone educate me about BPD?

I have or had a friend who has borderline personality disorder. He's a great guy and we got along pretty well, I can't say that I've ever had a friend that I was as comfortable around as I was with him. He was like a brother to me and whenever something happened or whenever I came across something interesting, I'd usually think "X would love this", he was the friend at the forefront of my mind.

We had started talking to each other at a point when neither of us had many friends, so we were each other's go to, time went by and we both made some more friends of our own but at some point, we started talking less and less and I'd be waiting on a reply from him for days or even weeks, there were times when I'd wonder what exactly I was staying around for. He's mentioned that he has issues with keeping relationships, he sometimes gets carried away when a new person comes around and they would 'consume' him for lack of a better word. I knew that he was spending more time with his other friends and he even mentioned it to me. There was another time when I told him about another friend I had made because I was really happy to have met that person and he got jealous I guess, said how I didn't need him any more and tried to leave and push me away. At the time, I said I wouldn't stop him if he really wanted to leave, he couldn't be the only friend I had, but if he wanted to, he could take some time to cool off and then we could talk. He agreed with that and so we talked about it a few days later.

Sometimes I wouldn't hear from him for a while but then he'd come around because he messed up somewhere or something happened and he needed some advice. I didn't mind for a while but then it felt like that was all I was good for. So at some point, I built up my resolve and left. He noticed and brought it up, said sorry, don't know if he even knew what he was apologizing for but it ended, I was tired and didn't feel like pushing anymore. But I still worry. A part of me says that he didn't mind the loss at all and that he's completely fine but another part can't help but worry. I don't think I truly understood what was going on in his head and it's already too late for both of us but I'd still like to learn about it if there's anyone willing to educate me