Decker/Ilia have more in common with Riker/Troi. With Chekov and Irina it was a case of he was too serious, and she was an Academy dropout. Irina was a girl who got away, but hardly the love of Chekov's life.
I believe that Riker and Troi's relationship and separation was more like Kirk and Janice Lester's. Where the man made the decision to leave the relationship in order to pursue his career.
Decker and Ilia was more Decker made the decision to leave the relationship to avoid being "enslaved" by engaging in a sexual affair with Ilia. Decker left not for the sake of his career, but to protect himself and his independence.
Decker's motivations were very different than Riker's or Kirk's.
The Chekov and Irina situation isn't remotely similar to the others. My impression is that she left him, so that she could follow her own path. A path that didn't include Chekov.