You guys have just exited the honeymoon phase of the relationship, and he's already latching onto feelings for another girl? After the first year to year and a half things cool down, that's normal for an LTR. You get comfortable around your partner, there's a deeper attraction that isn't just that fiery new love feeling. Getting crushes is also normal to an extent, but you have to avoid the crush and squash it instead of nurturing it (which is what it sounds like he's doing). And also getting a crush that quickly out of the honeymoon phase might sounds like he isn't appreciating the normal LTR feelings, and instead is chasing the new love energy.

I think you should talk to him about it before jumping on the breakup train. Give him a chance to tell you his side of the story. Tell him you went to find a poem in his journal but the last entry caught your eye, and it really hurt your feelings. See how he responds. If he doesn't want to put in the energy to maintain a LTR, it's going to crumble no matter how hard you try.