Every time I say goodbye to my boyfriend, I feel really sad and end up in tears. Why is that?

As I mentioned in the title, I (20F) always feel sad and end up crying whenever I say goodbye to my boyfriend (24M), and I’m not entirely sure why or how to address it. We’ve been together for over a year now, and I’ve never been happier in a relationship; I know he feels the same way. We rarely have any issues, and when we do argue, it’s usually over trivial matters. I recognize that I struggle with attachment and trust issues, stemming from my parents’ divorce when I was young and my father’s dishonesty. Despite having complete faith in my boyfriend, I can’t pinpoint why I feel this way when it’s time to part. I’ve tried picking up new hobbies to distract myself when we’re apart, and I do fine until I have to say goodbye and board the train, even though I know I’ll see him again in a few days. I’m just seeking some insight into why I feel this way and how I can improve. My boyfriend and I have discussed it, and he’s supportive; he just doesn’t want to see me upset because he wants me to be happy. Thank you to anyone who can offer advice!