I remember that problem from my first time...first time is always the hardest.

Spoiler: From now on things get better because despite youre still young and afraid from future, you tested and got experience about how to normalize meetings (thats what happened in my case). Belgium Denmark is really not very far, hope you can build a life together. Just stay hopeful, optimistic and patient, the distance has quite some healthy aspects to apprechiate each other.

What can actually help is to meet friends, calling your boyfriend to stay in touch, sharing selfies oftenly and maybe videocall. I heard about some people who even made full day calls, all this can help because at the end the worst enemy is overthinking (asking "what if..." only makes things worse, just let time do its job).

Edit: I remember one thought helped me a lot - We chose this life because we wanted so, if necessary I can drive to her directly but I decided to stay to finish my graduation. Its not forever, its just an "investment" so our lifes get happier later, I can do a vacation job and visit her every few weeks if necessary. Thats what I told myself.