DEAR MET, MY BOYFRIEND IS SOMEONE’S MAN

I met my bf at school. We started communicating about school work, we help each other out and so on. We got friendly then he wanted us to be in a relationship, I refused because he already has a gf. But we kept talking none the less about school life and so on. Eventually I ended up liking him too. When he asked the last time for us to get involve I said ok. My friends were against it because they say I might end up getting hurt because he has a girl. I told them I wouldn’t go so deep I’ll built a wall if it starts to crumple I’ll leave. We’ve been good we talk everyday about any and everything, we still help each other with school. I live a thousand miles away we don’t spend time much we mostly see each other at school. But we try not to let it affect us. Sometime it gets to me and i’d argue with him for no reason but he always calms me down. Despite the distance. My wall is crumbling so I know it’s time for me to leave. But my heart nor my soul doesn’t want to. I know my friends warned me so I don’t talk to them about it, but I’m really just idk. What do you think?