What do I get so depressed at night?

So my boyfriend is up at college and he's a golfer and lives 3 hrs away. I haven't seen him in 3 weeks and he's coming home this weekend tho... But lately I've been getting really depressed at night or sad or even mad... When he goes to hang out with his guy friends or just going to hit range balls it's like I don't want him to hang with anyone else if I can't be there... I know that's selfish and I don't want to be... But it just scares me bc the college he goes to is a big part school he doesn't do drugs or drinks but uk I still get worried. But again I just don't understand why I get so depressed at night?... Any help would be nice and no rude comments plz ?