First off, I'm sorry if I shouldn't post here because I'm not diagnosed so there's no way of telling if I even have BPD, but I strongly relate to most of the stuff people say on here so I thought you guys might understand.
Today, my friend who I used to be in a relationship with, told me that a female friend of his was offering to accompany him to a concert he didn't want to go to on his own. He'd already asked me to go with him weeks prior, but I can't because it's a weekday and I have to go to school. The concert is in Berlin (I'm German) which is about 7 hours from the town I live in. 
I instantly got incredibly jealous. It sounds idiotic to even admit to this because I know it doesn't make any sense, but I immediately felt like he was going to replace me or something? I'm aware of how irrational the whole thing must sound and I didn't let him know how I felt about the situation because I didn't want to start another completely pointless fight with me being in the wrong.
And this isn't the first time something like this has happened. I tend to get jealous as soon as one of my friends mentions taking part in an activity that doesn't involve me and it's the thing I hate the most about myself. I never let people know how jealous I am because I grew up thinking jealousy was one of the worst things possible? It's really hard to explain. It usually ends in me distancing myself from a person in an extreme attempt to not seem clingy or overly attached.
Also I'm sorry for my shitty English, I'm not a native speaker and have no idea how to use commas.