Long distance with my first boyfriend

Lately my I’ve been on an emotional roller coaster from being calm to sad to jealous, I can’t control how I feel as much as it is my choice to feel this way. I’ve been thinking a lot, perhaps too much, between my boyfriend and I and if our relationship will last. We've been dating for almost a year and he had recently move back to his home country.
I know that it’s silly to think so far into the inevitable future where anything could happen, but I can’t stop myself from thinking and asking myself all these questions. What if he finds another girl in his country that has as much or even more qualities than I do? What if he loses feelings for me because he’s so use to not having me there? Is he still looking at HER pictures on his chatting app and saving them? Why does it still hurt when he brings up HER name (she existed two years before we met)? Is he looking at other girls and having fantasies about them? Does he still wish that he could talk to her and mend their broken friendship and perhaps more? Why am I even questioning his actions? Why can’t I trust him? Am I too sensitive? What did I do wrong or what am I lacking in quality to have him keep doing these things that makes me question my trust for him? I shouldn’t feel jealous when he hangs out with other girls, since he has his own personal space and friends but why do I find it difficult for me to not think otherwise? Why am I thinking so much?