The problem of the throwaway boyfriend
In the first 10% of my novel, my MC has a boyfriend. MC is accepted into the Space Corps (or he's summoned to fight Troy - the particulars don't really matter), boyfriend is sure he'll wait the required X years.
MC starts training, and already the forced separation, the change in their respective lifestyles, the separate new experiences - all of it draws them apart. There's friction, eventually they break up.
From a Watsonian perspective, the MC finds himself sacrificing a relationship that was important to him, for his chosen path. It's painful, he struggles with it, he tries to keep both, eventually he accepts it.
From a Doylist perspective, of course they're going to break up. From the beginning, a long-distance relationship isn't very interesting - having the MC meet someone new and exciting a bit further into the story has much more "meat". So it's just a question of when. Once the conflict appears, it's therefore rather obvious how it ends.
I'm struggling to maintain tension in the struggling-relationship period. On the one hand, the conflict needs time to develop - they wouldn't just break up at the first sign of trouble. That would feel unrealistically abrupt, and devoid of the related internal struggle for the MC. On the other hand, it is clear that they're going to break up. Not clear to the characters, but clear to a savvy reader. Since it's clear how the conflict is going to end, why would a reader be invested in it? How do I prevent a "get on with it, break up already!" reaction?
A particularly important element: boyfriend is not a schmuck. Rather, they both, and MC in particular, underestimate how hard the ordeal is going to be. Which is an important recurring theme I want to set up early.