how can I handle strong emotions in a public place like school?

im under 18 and diagnosed with bpd, so you can imagine the combination of hormones and bpd mood swings that get me. I try not to let it affect my relationships, but it did today.
while I was having lunch in my school with my friend, she said she was going to go find a room to record in, and I agreed. she left me alone for a while, almost twenty-thirty minutes to the point id began to shift from happy to extremely frustrated. I got up to try find her, overthinking about how she likely got annoyed with me and wanted to be with anyone else. so, when I look through the glass door of a classroom to see her with someone else, I immediately crashed out. all I could think was that she'd abandoned me, so I stormed into the classroom. I yelled "fuck you" loudly and told her she was a bitch for leaving me. I got so angry, I just went home. the second I left school, I felt so much guilt and sadness hit me.

I thought about how badly I handled the situation, and how my efforts to handle bpd better had gone to waste. my question is: how can I handle my emotions better? how can I not explode like that again? and am I a selfish person, or is it just the bpd causing that scenario? thank u