I am overthinking that my (26F) bf (27M) will leave me for someone exciting?

I (26F) love my bf(27M) alot. We are each other's first relationships and best friends. We've been dating for more than a year and now it's the period where we don't have much to talk on calls. Soon we will be coming into long distance of two years as he will be going for his job training. I fear that at the training he'll meet someone more exciting as newness is fun and he'll get to be with his fellow trainees 24*7 as they'll live together. He always gives me reassurance but somehow I'm scared of being replaced and I can see these patterns of boredom setting in. What can I do?